The first step towards an Anglo-Norman conquest of Ireland was made by Henry II of England, who is said to have obtained in 1155 a bull (official document) from Pope Adrian IV authorising him to take possession of the island, on condition of paying to the papal treasury a stipulated annual revenue. This bull is thought to have been a forgery. 

In any event, nothing was done until Dermot MacMurrough, the deposed king of Leinster, sought refuge at Henry's court and obtained permission to enlist the services of English subjects for a recovery of his kingdom. Dermot returned to Ireland in 1169 with foreign mercenaries and numerous Irish allies, and succeeded in recovering part of his former territories as well as capturing Dublin and other towns on the east coast. After his death the succession to the kingdom of Leinster was claimed by his son-in-law Richard Strongbow, 2nd Earl of Pembroke. 

In 1171 Henry, with a large army, visited Ireland, received homage from the principal Norman leaders and from the Irish High King, Rory O'Connor, and granted charters to the Normans authorising them, as his subjects, to take possession of portions of the island. The chief Anglo-Norman adventurers, however, encountered formidable opposition before they succeeded in establishing themselves on the lands that they claimed. The government was entrusted to a viceroy, and the Norman legal system was introduced into such parts of the island as were reduced to obedience to England.  

The young Prince John, later King John of England, was sent by Henry into Ireland in 1185, but the injudicious conduct of his council provoked disturbances, and he was soon recalled to England. John made a second expedition to Ireland in 1210 to curb the refractory spirit of his Norman barons, who had become strong through alliances with the Irish. 

During the 13th century various Anglo-Norman adventurers succeeded in firmly establishing themselves in Ireland, either by assisting or suppressing native clans. The Fitzgerald clan acquired power in Kildare and East Munster; the Le Botiller, or Butler, in West Munster; and the de Burgh in Connaught. 

After the Battle of Bannockburn in 1314, Edward Bruce, the younger brother of Robert Bruce, King of Scotland, invaded Ireland and attempted unsuccessfully to overthrow the English there. The pope, at the instigation of England, excommunicated Bruce and his Irish allies. Although Bruce's enterprise failed, his invasion highlighted the decline of English power in Ireland. 

The descendants of the most powerful Anglo-Norman settlers in Ireland gradually became identified with the native Irish, whose language, habits, and laws they adopted to an increasing extent. To counteract this, the Anglo-Irish Parliament passed, in 1366-1367, the Statutes of Kilkenny, decreeing excommunication and heavy penalties against all those who followed the custom of, or allied themselves with, the native Irish. 

This statute, however, remained inoperative; and although Richard II later in the 14th century made expeditions into Ireland with large forces, he failed to achieve any practical result. The power and influence of the natives increased so much at the time of the Wars of the Roses that the authority of the English Crown became limited to the area known as the English Pale, a small coastal district around Dublin and the port of Drogheda. In the Wars of the Roses, the struggle in England between the houses of York and Lancaster, Ireland supported the ultimate loser - the House of York.