or Diarmaid Macmurchada
died May 1, 1171
Irish king of Leinster (1126–71).
He faced a number of rivals in claiming the throne of his father, Enna, and he asserted his authority by killing or blinding 17 rebel chieftains (1141). He abducted another Irish king's wife in 1153, beginning a bitter feud in which he was driven from Ireland (1166). He returned with the backing of Anglo-Norman lords (including the earl of Pembroke), and in 1170 they captured Dublin. Pembroke married Dermot's daughter and became ruler of Leinster after his death. Dermot's appeal for Norman help in settling an internal dispute thus proved instrumental in the Norman conquest of Ireland.