Dermot MacMurrough

Diarmait Mac Murchada anglicized as Dermot MacMurrough (died 1 January 1171) was the King of Leinster. 

Ousted as King of Leinster, he invited King Henry II of England to assist him in regaining the throne. The subsequent invasion led to Henry becoming Lord of Ireland himself, and marked the beginning of eight centuries of English dominance.