(d.1072), king of Leinster and contender for the kingship of Ireland. On the death of the king of Leinster, Murchad mac Dúnlaing, at the battle of Mag Mulchet (in the territory of the Loígis) in 1042, the Uí Dúnlainge dominance of Leinster ended and Diarmait's claim to the kingship was consolidated. From 1048, Diarmait engaged in the affairs of other provinces. He led an army into Mide (Meath) in 1048 in revenge for raids on Leinster by Conchobar ua Máel Sechlainn (Conor, grandson of Malachy), king of Mide (d. 1073). This action prompted the high king Donnchad mac Briain (Donough, son of Brian) to seek Diarmait's submission. By c.1054 Diarmait controlled Dublin, which he entrusted to his son Murchad (Murrough) (d. 1070), who acted as king in his absence. In alliance with Áed Ua Conchobair (Hugh O'Conor), king of Connacht, and with Toirdelbach Ua Briain (Turlough O'Brien), he challenged Donnchad mac Briain's authority until the high king was ousted and went to Rome in 1064. Diarmait's ally Toirdelbach Ua Briain then assumed the kingship of Munster. With the support of the Munstermen and the Osraige, Diarmait assisted Áed Ua Ruairc (Hugh O'Rourke) (d. 1087) in defeating Áed Ua Conchobair in 1067. Diarmait was killed at the battle of Odba (?Navan, Co. Meath) in 1072 by Conchobar ua Máel Sechlainn, king of Mide.
From The Oxford Companion to Irish History in Oxford Reference.
Subjects: European History.