By 1167 MacMurrough had obtained the services of Maurice Fitz Gerald and later persuaded Rhys ap Gruffydd Prince of Deheubarth to release Maurice's half-brother Robert Fitz-Stephen from captivity to take part in the expedition. Most importantly he obtained the support of the Earl of Pembroke Richard de Clare, known as Strongbow. The first Norman knight to land in Ireland was Richard fitz Godbert de Roche in 1167, but it was not until 1169 that the main forces of Normans, Welsh and Flemings landed in Wexford.
Within a short time Leinster was regained, Waterford and Dublin were under Diarmait's control, and he had Strongbow as a son-in-law, and named him as heir to his kingdom. This latter development caused consternation to King Henry II of England, who feared the establishment of a rival Norman state in Ireland. Accordingly, he resolved to visit Leinster to establish his authority. The Papal Bull and Henry's invasion: Pope Adrian IV (the first English Pope, in one of his earliest acts) had already issued a Papal Bull in 1155, giving Henry authority to invade Ireland as a means of curbing ecclesiastical orruption and abuses. Little contemporary use, however, was made of the Bull Laudabiliter since its text enforced papal suzerainty not only over the island of Ireland but of all islands off of the European coast, including England, in virtue of the Constantinian donation. The relevant text reads: "There is indeed no doubt, as thy Highness doth also acknowledge, that Ireland and all other islands which Christ the Sun of Righteousness has illumined, and which have received the doctrines of the Christian faith, belong to the jurisdiction of St. Peter and of the holy Roman Church". References to Laudabiliter become more frequent in the later Tudor period when the researches of the renaissance humanist scholars cast doubt on the historicity of the Donation of Constantine. Henry landed with a large fleet at Waterford in 1171, becoming the first King of England to set foot on Irish soil. Both Waterford and Dublin were proclaimed Royal Cities. Adrian's successor, Pope Alexander III ratified the grant of Irish lands to Henry in 1172.
Henry awarded his Irish territories to his younger son John with the title Dominus Hiberniae ("Lord of Ireland"). When John unexpectedly succeeded his brother as King John, the "Kingdom of Ireland" fell directly under the English Crown. Henry was happily acknowledged by most of the Irish Kings, who saw in him a chance to curb the expansion of both Leinster and the Hiberno-Normans. This led to the ratification of the Treaty of Windsor (1175) between Henry and Ruaidhri. However, with both Diarmuid and Strongbow dead (in 1171 and 1176), Henry back in England and Ruaidhri unable to curb his nominal vassals, within two years it was not worth the vellum it was inscribed upon. John de Courcy invaded and gained much of east Ulster in 1177, Raymond le Gros had already captured Limerick and much of north Munster, while the other Norman families such as Prendergast, fitz Stephen, fitz Gerald, fitz Henry and le Poer were actively carving out virtual kingdoms for themselves.