Laudabiliter was a papal bull issued in 1155 by Adrian IV, an Englishman, giving the Angevin Henry II of England the right to assume control over Ireland and apply the Gregorian Reforms to the Irish church. The Norman invasion of Ireland ensued in 1169–71 under the authority of this bull. Richard de Clare a Cambro-Norman knight from Pembroke in Wales was retained by the King of Leinster, Diarmuid MacMorrough as an ally in his fight with the Rudhri O'Connor, High King and Tighernan O Ruairc of Brefni to regain control of his Irish kingdom.
In the event, successive Kings of England, from Henry II (1171) until Henry VIII (1541), derived the title and the authority of the Lord of Ireland from Adrian's successor, Pope Alexander III.
After almost four centuries, following the declaration of the independence of the Church of England from papal supremacy and rejection of the authority of Rome, a new basis for the English monarch's legitimate claim to the rule of Ireland was needed: the Crown of Ireland Act 1542 therefore established a sovereign Kingdom of Ireland with Henry being given the title of King of Ireland. There has been some controversy over the authenticity of the Laudabiliter.
Read more about Laudabiliter: Papal Bull, The Bull Laudabiliter, Evidence For The Bull, Controversy, Divided Significance, Authenticity Debate, Papal Letter of 1311 and The Irish Kings' Remonstrance of 1317, Laudabiliter and The Kingdom of Ireland 1542–1555