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Gaelic Ireland (Irish: Eire Ghaidhealach) was the Gaelic political and social order, and associated culture, that existed in Ireland from the prehistoric era until the early 17th century. Before the Norman invasion of 1169, Gaelic Ireland comprised the whole island. Thereafter, it comprised that part of the country not under foreign dominion at a given time. For most of its history, Gaelic Ireland was a "patchwork" hierarchy of territories ruled by a hierarchy of kings or chiefs, who were elected through tanistry.

Ireland became Christianized between the 5th and 7th centuries. Pope Adrian IV, the only English pope, had already issued a Papal Bull in 1155 giving Henry II of England authority to invade Ireland as a means of curbing Irish refusal to recognize Roman law. Importantly, for later English monarchs, the Bull, Laudabiliter, maintained papal suzerainty over the island:  In 1166, after losing the protection of High King Muirchertach Mac Lochlainn, King Diarmait Mac Murchada of Leinster was forcibly exiled by a confederation of Irish forces under King Ruaidri mac Tairrdelbach Ua Conchobair. Fleeing first to Bristol and then to Normandy, Diarmait obtained permission from Henry II of England to use his subjects to regain his kingdom. By the following year, he had obtained these services and in 1169 the main body of Norman, Welsh and Flemish forces landed in Ireland and quickly retook Leinster and the cities of Waterford and Dublin on behalf of Diarmait. The leader of the Norman force, Richard de Clare, 2nd Earl of Pembroke, more commonly known as Strongbow, married Diarmait's daughter, Aoife, and was named tanaiste to the Kingdom of Leinster. This caused consternation to Henry II, who feared the establishment of a rival Norman state in Ireland. Accordingly, he resolved to visit Leinster to establish his authority.  Henry landed in 1171, proclaiming Waterford and Dublin as Royal Cities. Adrian's successor, Pope Alexander III, ratified the grant of Ireland to Henry in 1172. The 1175 Treaty of Windsor between Henry and Ruaidhri maintained Ruaidhri as High King of Ireland but codified Henry's control of Leinster, Meath and Waterford. However, with Diarmuid and Strongbow dead, Henry back in England, and Ruaidhri unable to curb his vassals, the high kingship rapidly lost control of the country. Henry, in 1185, awarded his younger son, John, the title Dominus Hiberniae, "Lord of Ireland". This kept the newly created title and the Kingdom of England personally and legally separate. However, when John unexpectedly succeeded his brother as King of England in 1199, the Lordship of Ireland fell back into personal union with the Kingdom of England.

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Pope Adrian IV, the only English pope, had already issued a Papal Bull in 1155 giving Henry II of England authority to invade Ireland