Diarmait or Diarmaid Mac Murchadha (later known as Diarmaid na nGall or "Diarmaid of the Foreigners"), anglicized as Dermot MacMurrough (1110–1 May 1171), was a King of Leinster in Ireland. Ousted as King of Leinster in 1166, he sought military assistance from King Henry II of England to retake his kingdom. In return, MacMurrough pledged an Oath of Allegiance to Henry, who sent troops in support. As a further thanks for his reinstatement, MacMurrough's daughter Aoife was married to Richard de Clare, the 2nd Earl of Pembroke and a Cambro-Norman lord, known as "Strongbow". Henry II then mounted a larger second invasion in 1171 to ensure his control over Strongbow, and since then parts of, or all of, Ireland has been ruled or reigned over by the monarchs of England.
In Irish history books written after 1800 in the age of nationalism, Diarmaid Mac Murchadha was often seen as a traitor, but his intention was not to aid an English invasion of Ireland, but rather to use Henry's assistance to become the High King of Ireland himself. He had no way of knowing Henry II's ambitions in Ireland. In his time, politics was based on dynasties and Ireland was not ruled as a unitary state. In turn, Henry II did not consider himself to be English or Norman, but a French Angevin, and was merely responding to the realities on the ground.
Gerald of Wales, a Cambro-Norman historian who visited Ireland in 1185 and whose uncles and cousins were prominent soldiers in the army of Strongbow, repeated their opinions of Mac Murchadha:
"Now Dermot was a man tall of stature and stout of frame; a soldier whose heart was in the fray, and held valiant among his own nation. From often shouting his battle-cry his voice had become hoarse. A man who liked better to be feared by all than loved by any. One who would oppress his greater vassals, while he raised to high station men of lowly birth. A tyrant to his own subjects, he was hated by strangers; his hand was against every man, and every man's hand against him."