While Richard ‘Strongbow’ de Clare was the first Norman to gain a foothold in Ireland, by 1171, King Henry of England had gotten involved and he and his knights carved out huge sections of eastern and southern Ireland for themselves. Other knights marched north and established a northern bastion at Carrickfergus, which became the seat of the Earl of Ulster.
Over time, however, Anglo-Norman rule ebbed and flowed. In fact, as the centuries progressed, it ebbed more than flowed, such that by 1500, the descendants of the original conquerors were almost completely assimilated into the native Irish clans. It reached a point such that Henry VIII offered amnesty to all lords in Ireland regardless of ethnicity, provided they surrendered their lands to him (to receive them back immediately by royal charter).
Unfortunately for Ireland, after two hundred years of being mostly ignored by the English crown, the Tudors decided that the time had come to ‘pacify’ and ‘Anglicize’ the island to bring it under more direct English control. The country’s offenses were remaining Catholic while England had gone Protestant, and the continued existence of clans and kingdoms outside of the standardized English system. The “Old English” families, as the former Anglo-Norman families were called, were viewed as no better than the native Irish. All were stripped of power and forced off their lands by new rulers and imported settlers from England, Scotland, and Wales, who were, of course, Protestant as well.
“The first and most important result of the conquest was the disarmament of the native Irish lordships and the establishment of central government control for the first time over the whole island; Irish culture, law and language were replaced; and many Irish lords lost their lands and hereditary authority. Thousands of English, Scottish and Welsh settlers were introduced into the country and the administration of justice was enforced according to English common law and statutes of the Parliament of Ireland.
As the 16th century progressed, the religious question grew in significance. Rebels such as James Fitzmaurice Fitzgerald and Hugh O’Neill sought and received help from Catholic powers in Europe, justifying their actions on religious grounds . . . Under James I, Catholics were barred from all public office … the Gaelic Irish and Old English increasingly defined themselves as Catholic in opposition to the Protestant New English … By the end of the resulting Cromwellian conquest of Ireland in the 1650s, the “New English” Protestants dominated the country, and after the Glorious Revolution of 1688 their descendants went on to form the Protestant Ascendancy.” https://en.wikipedia.org/wiki/Tudor_conquest_of_Ireland
Far more than the initial Norman conquest of Ireland, it is in the Cromwellian conquest where the roots of the deep resentment of the Irish people towards the English lie, as well as the source of the campaign for independence that marked the eighteenth through twentieth centuries.