In 1166, after feuding with his neighbours, he was defeated and banished by the Irish High King. He sought support from Henry II of England and obtained the aid of Richard de Clare, Earl of Pembroke (‘Strongbow’), offering him his daughter in marriage and the succession of Leinster. Dermot regained his kingdom in 1170 and after his death Leinster became an English fief and Henry II began to establish English dominance in Ireland.