The immediate cause of invasion was due to the brutal savagery of Dermot Macmurragh, the king of Leinster, who had outraged even the loose morals then prevalent among the tribes. Deposed from his kingdom by Roderick O'Conor, the overlord of Ireland, Dermot had sought refuge in England, offering allegiance to Henry , and asking assistance against his enemies. King Henry, though he had already had some intentions of descending on Ireland, was too much occupied with his own kingdom just then to proceed in person, but he granted permission for his knights to aid Dermot. The Norman knights were eager to enlist, the opportunity of fighting, combined with a prospect of glory and land, holding out strong inducements. The leaders of the invading party were Richard de Clare, Earl of Pembroke (usually known as Strongbow), Robert Fitzstephen, and Maurice Fitzgerald. Accompanying the Normans as their chronicler was the Welsh priest, Giraldus Cambrensis, or Gerald of Wales, from whose account, brilliant and fascinating, but far from impartial, we gain our chief knowledge of this period.

Fitzstephen landed in Ireland in 1169 with a small force of some 500 men, but all of them efficient and well-trained soldiers, equipped with the best weapons of the time. Joined by Dermot and his tribesmen, the Normans attacked Wexford, which they took after some difficulty, and then marched inland to meet the men of Ossory, who had gathered an army to resist the invaders. Enticing the Celtic levies into the plain, Fitzstephen charged them with his horsemen and cut them to pieces. Shortly afterwards, terms were made with King Roderick, who restored Leinster to Dermot. The following year Strongbow arrived with another Norman contingent, took Waterford, in which city he married Eva, the daughter of Dermot, whose heir he now became—and then proceeded northwards towards Dublin. The Danish castle at Dublin seems to have been destroyed after the Battle of Clontarf, for the city soon fell to Strongbow's men, the Danes under their leader Hasculf taking flight in their ships. Troubles now began for the hitherto conquering Normans. Dermot had died, leaving his kingdom of Leinster to Strongbow, and enemies were gathering everywhere. Hasculf Thorgilsson returned with a fleet, storming Dublin from the sea, while shortly afterwards Roderick besieged it from the land. Surrounded by foes, the garrison determined on a desperate sally, which so surprised the Irish that they fled panic-stricken. Strong-bow then returned to England to pacify King Henry, who was growing jealous of his powerful subject.

Owing to the independence of the Irish Church, it had come into disfavour with Rome, who considered it quite heretical, and who therefore encouraged Henry in his proposed conquest of the country, in order that it should be brought back into complete submission to Papal authority. Pope Adrian , being an Englishman, possibly entertained some national feeling when he issued a Bull permitting Henry to enter Ireland and "to execute therein whatever shall pertain to the honour of God and welfare of the land." Armed with this authority, Henry landed at Waterford in the autumn of 1171, bringing with him so large an army that resistance was practically impossible. A great majority of the chiefs submitted, Roderick being among their number. Henry entertained them lavishly at Dublin, where he instituted a rough kind of feudalism, the chiefs rendering their land to him to receive it back again as vassals. To the Irish, King Henry took the place of their overlord, who had possessed mere nominal power, very different to that of a feudal king, who claimed a man's lands when he no longer did service for them. This distinction was the cause of much trouble in the future.

With the natural governing capacity of the Normans, Henry proceeded to plan a new system for Ireland. The country was divided into counties, and English law instituted, with circuit judges. A synod of the church was held at Cashel, where the Irish Church was brought into line with the English, and where Henry was acknowledged as "Lord and King at the hand of Providence." It is not known how many of the Irish clergy accepted this, but it is certain that the Irish Church was very little altered.

After about six months in Ireland, Henry was recalled suddenly to England on learning that papal legates were on their way to inquire concerning the murder of Becket. According to Sir John Davies, a Tudor statesman, Henry "departed from Ireland without striking one blow, or building one castle, or planting one garrison among the Irish," and therefore left the country to an inevitable return to disorder. It is a commonplace that a well-organized country under one king is easier to conquer than a wild country split up among different rulers. The Norman Conquest had been purely superficial, the chiefs surrendering in name, but remaining independent in their own inaccessible districts, beyond the reach of law, unless accompanied by an armed force. Dense forests, swamps, and mountains prevented the easy movements of disciplined troops, so that royal authority would have been difficult to maintain in any case; but had Henry thoroughly subdued Ireland, leaving no corner undisturbed, and then left a strong force to carry out the orders of a powerful central government, the history of the country would have undoubtedly been changed and centuries of unhappiness been spared. Instead of this a weak colony was left, settled on lands confiscated from natural owners during the Norman Conquest, the colonists unable to do more than hold their own; while the Celtic population remained in a state of wild disorder, far removed from that ideal of perfect conquest held by Davies, "which doth reduce all people thereof to the condition of subjects; and those I call subjects which are governed by the ordinary laws and magistrates of the sovereign."

Though unable to attend to Ireland himself, Henry sent over his youngest son, Prince John, a mere boy, to keep order in Ireland. But the Prince and his frivolous courtiers only created tumult, annoying both the Celtic chiefs and the Anglo-Norman settlers by their careless and insulting behaviour, till the young Prince was recalled. When John became king his attitude towards Ireland changed; he ordered his governor to build Dublin Castle, there being no safe place for the treasury, and came himself in 1210 to extend his authority and to consolidate the conquest commenced by his father. Again the Celtic chiefs made submission to the English king, who portioned out more of the land into counties, almost up to the Shannon, and then, leaving commands for castles to be built, he returned to England. Ireland remained almost unaltered by his visit, for the Celtic chiefs did as they liked in their own territory, while the Norman nobles, settled on the fertile eastern part of Ireland, maintained a condition of feudal tyranny. Impartial law did not exist, there being no force to carry out its dictates, and anarchy continued to prevail in Ireland.

A little over a century after King John's visit, the English were almost driven out of Ireland. The O'Neills invited Edward Bruce, the younger brother of King Robert of Scotland, to attempt the conquest of Ireland. Landing with a large army in 1315, Edward Bruce was joined by most of the Irish chiefs, and was crowned king at Carrickfergus. King Robert came over to assist his brother, and together they spread over Ireland, carrying destruction with them. The English were utterly unable to resist so powerful a force, and were in imminent danger of annihilation; but Edward Bruce's own tactics brought him to disaster and death. He had burnt and pillaged the country mercilessly, revenging Scotland's wrongs on the English settlers, till famine and pestilence stalked the land. An English army was sent over to help the colonists, who, making a supreme effort, overthrew the Scottish army at Dundalk, where Edward Bruce was slain. The English had been saved, but at a severe cost, the colonists long remaining enfeebled and powerless.

From this time until the coming of the Tudors the power of the English in Ireland began to decline. The fundamental cause of this decline was the feebleness of the original conquest, so fatally easy to accomplish at first, but never really completed. The lack of English colonists, who remained few in numbers, and the feudal oppression of the Anglo-Norman nobles, united to prevent the spread of English law and authority. But the greatest misfortune was the absence of the king. Royal power was a blessing to the general population in the Middle Ages, the king acting as a restraint upon the insurgent barons, and when the kings were feeble or absent their subjects suffered. For nearly two centuries after King John's visit, no English king had landed on Irish shores, the unhappy island being left a prey to greed, tyranny, and lawlessness. No attempt at an organized policy was carried on with regard to Ireland, an evil from which she still suffers, each Viceroy acting upon the system which seemed to him best. Some came to enrich themselves at the expense of Anglo-Irish and Celt alike, others to subdue the power of the great feudal nobles, while others maintained the feudal power in its full force. These constant changes of policy naturally resulted in perpetual feuds, animosities, and general disorder. The English Pale, that district peopled by the English settlers and ruled by English law, became more and more restricted. At one time it had almost reached the Shannon, then it was limited to the four counties of Meath, Louth, Dublin, and Kildare, and finally it was confined to a mere strip along the coast, twenty miles wide, stretching from Dundalk to the mountains of Wicklow.

In Celtic Ireland, consisting of nearly three-quarters of the whole island, the impression of the Norman Conquest, always slight, was almost effaced. The Irish chiefs lived in their old lawless way, only influenced for the worse by the feudal system, which had broken up the family tribal idea, leaving the chief the arbitrary head of his clan. Instead of a communistic system, all members of a clan possessing some claim to the land, the chief had obtained almost the sole rights, thus lowering his free people to the position of subject vassals. But along with this veneer of feudalism many old tribal and barbarous customs remained.

Two great families, both originally Anglo-Norman, swayed the destinies of Ireland. The Butlers, ancestors of the ducal house of Ormond, owned vast possessions, ruling them with almost regal power. The Fitzgeralds, who included the House of Kildare and the Desmonds, reigned from the wild district in the west among the hills and lakes of Kerry up to the county of Kildare, on the edge of the Pale. Both families were received at the court of the English Kings, though they kept the land in a state of feud, their adherents, like the Border raiders, being engaged in a life of freebooting expeditions for plunder and pillage. A certain amount of civilization had arisen in the island, a considerable trade was carried on between Galway and Spain, and stately abbeys and strong castles were built. But while England was attaining greater freedom and growing in power, Ireland was torn by petty wars and hindered from the progression which only peace can produce.

Those Anglo-Norman nobles who lived far removed from English influence gradually conformed to Irish ways and methods, becoming "more Irish than the Irish themselves," using native dress and speech, sinking to the level of the less civilized Celts. The Desmonds soon became classed among these "degenerate Englishry," as their land lay almost solely among the native Irish, with only a small sprinkling of English settlers.

The worst sufferers under these unsettled conditions were, naturally, the lowest classes, crushed by the all-powerful nobles and Celtic chiefs, who lived in rude luxury while the tillers of the soil existed in wretchedness. Anglo-Irish baron and Irish chief continued to raid upon one another, gaily carrying off cattle and burning the crops upon which the poor labourer depended.

Not only had the invading English settled upon the land of the Irish, but they had brought their own Church with them; for the Irish regarded the regular Papal system inaugurated at the Synod of Cashel as quite different to their native Catholic Church. Racial dislike kept the two Churches separate, though the points of difference were really trivial ones. Both Churches were alike in failing to produce men of distinction in learning or piety, no colleges or Universities were founded to be the nursery for wise leaders of the people, and general ignorance prevailed. As for the law, it differed like the Churches. English law was administered in the Pale in an extremely partial form, the native Irish being outside its justice. If an Irishman murdered an Englishman, he was condemned to death; but if the crime were committed the reverse way, no penalty was inflicted on the Englishman. A shadowy Parliament, meeting at Dublin, Drogheda, or Kilkenny, summoned at the arbitrary pleasure of the English king, carried out the wishes of the governing powers. No representatives of the native Irish sat in this Parliament. The most bitter Act passed by this unrepresentative assembly was that known as the Statute of Kilkenny. Lionel, Duke of Clarence, son of Edward , had come over to Ireland as Lord Lieutenant, and also to obtain the earldom of Ulster, which had become his through his marriage with the heiress. At his bidding the Parliament met at Kilkenny, and proceeded to concoct legislation to prevent the mingling of the two races which was then going on, with the ultimate idea of exterminating the Irish altogether. According to the Statute of Kilkenny, intermarriage with the Irish was to be regarded as treason, the using of the Irish language or dress by an Englishman involved imprisonment and forfeiture of property, and no Irishman was to hold any office whatever in the government. Owing to the enormous predominance of the Irish people and the growing feebleness of the English colony, this aggressive law never became effective.

After an absence of nearly 200 years, an English king again set foot upon Irish shores in 1394, of course accompanied by a large army to crush his Irish subjects; for Ireland, as one recent writer puts it, was an "unhappy country which for centuries never saw its kings but as invaders." A very serious rebellion, headed by a certain chieftain named Art MacMurrough, had at last forced pleasure-loving Richard to leave his luxurious Court, with the idea of gaining glory by fighting the Irish. If he was to remain Lord of Ireland any longer, it was necessary for him to come at once, and to come backed with the power of England. Art MacMurrough was lording it over the colonists, extracting a tribute from them known as "Black Rent," and no one was able to resist his furious onslaught. Richard arrived at Waterford with a vast army, sufficient to overawe any Irish forces, and the same story repeated itself as on the two previous occasions of a royal visit. The Irish chieftains submitted, Richard entertained them at Dublin, and honoured four of them with knighthood, one being Art MacMurrough himself. But no sooner had Richard returned to England than rebellion broke out again, the Viceroy, Roger Mortimer, Earl of March, being slain. In great anger Richard again descended on Ireland in 1399, but this time his army did not frighten Art MacMurrough, who kept his own small force hidden in the woods, refusing to be drawn into the open. Sickness spread in the English camp owing to the wet weather, obliging Richard to open negotiations with MacMurrough, and terms were still unarranged when news from England caused Richard to leave Ireland to its fate. His dangerous rival, Henry of Lancaster, had returned from exile, and had landed in Yorkshire, where the English were flocking to his banner. Richard collected his army and sailed across to Milford Haven, to find that his cause was already lost. Within a short time he was a captive, to suffer an obscure death some years later. English aid being always spasmodic, the colonists of the Pale had again to submit to Art MacMurrough's exactions, and pay his "Black Rent" until his death, in 1417.

Left to the mercy of various viceroys by the kings of the House of Lancaster, who were either busily engaged in maintaining their throne or trying to add France to their domains, Ireland became more and more given up to personal feuds and general disorder and misery. The Wars of the Roses, which for ever broke the power of the English nobility, left the commonalty of England practically untouched, the ordinary course of commerce and justice being little interrupted. But the wars of the rival Houses of York and Lancaster brought fresh cause of famine, bloodshed, and destruction, to the Irish people. The Duke of York had been viceroy in 1449, when he had won much love by his justice and conciliatory manners. During the contest many English from the Pale and Anglo-Irish nobles fought in England, and carried on the spirit of feud in Ireland till the English colony was almost extinguished. The Fitzgeralds were Yorkists, and in the triumph of their party rose supreme above the Butlers, who had supported the Lancastrians. In gratitude for their services, the head of the Fitzgeralds was appointed Viceroy by Edward , and for many years his family was predominant over the fortunes of Ireland.