He is believed to have been the commander of the Irish fleet that supported Rhys ap Tewdwr at the Battle of Mynydd Carn.
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[2] but he was eventually captured by Muircheartach.
[3] He married Mór Ua Conchobair, daughter of Ruaidrí Ua Conchobair [(anglicised Roderic O'Connor), called Ruaidrí na Saide Buide], King of Connacht (died 1118) and had two sons:[4] He had two other sons by unknown wives or mistresses: Source:[5]