Irish king of Leinster whose appeal to the English for help in settling an internal dispute led to the Anglo-Norman invasion and conquest of Ireland by England.

After succeeding to the throne of his father, Enna, in 1126, Dermot faced a number of rivals who disputed his claim to the kingship. He established his authority by killing or blinding 17 rebel chieftains of northern Leinster in 1141. In 1153 he abducted the wife of Tiernan O'Ruark, king of Breifne (modern counties of Leitrim and Cavan).

A bitter feud ensued, and in 1166 Dermot was driven from Ireland. King Henry II of England then granted the exiled ruler permission to enlist the aid of several Anglo-Norman lords of south Wales, notably Richard FitzGilbert, 2nd earl of Pembroke (Pembroke, Richard FitzGilbert, 2nd Earl of), who was given the nickname Strongbow. Returning to Leinster in 1167 with an advance party of Anglo-Normans, Dermot established a foothold there. Pembroke arrived in August 1170, and Dermot then helped the invaders capture Dublin. Dermot married his daughter Eva to Pembroke, and at Dermot's death Pembroke succeeded as ruler of Leinster.