Career
He is believed to have been the commander of the Irish fleet that supported Rhys ap Tewdwr at the Battle of Mynydd Carn. In 1093, he reconciled with Muirchertach after recognizing his claim and was installed as dux of Waterford. After taking the throne, he banished Muircheartach and soon was involved in a four-year struggle for control of Munster.
In 1116, he attacked Muirchertach and Brian Ua Briain "in violation of a mutual oath on the relics of Ireland" besieging Limerick.
This resulted in Muirchertach making peace with Diarmait but he was eventually captured by Muircheartach. He later died in Cork in 1118 and in the same year, Toirdelbach Ua Conchobair invaded Munster and partitioned the land between the sons of Diarmait Ua Briain and Tadg Mac Carthaig, Conchobar Ua Briain of Thomond and Toirdelbach Mac Carthaig of Desmond respectively.