Ua Briain, Diarmait (d. 1118) was a son of the Munster king, Tairdelbach ua Briain (qv) (d. 1086) and of Dubchoblaig, daughter of Lorcán, of the Leinster dynasty of Uí Chennselaig. Already politically active during his father's reign, in 1080 he sailed to Wales from where he carried off great spoils. About this time, too, he may have been appointed governor of Waterford by his father. On Tairdelbach's death in 1086, the kingdom was divided between Diarmait and his two half-brothers, Muirchertach (qv) and Tadc. The latter's death soon afterwards upset this arrangement: banishing his remaining brother, Diarmait, from Munster, Muirchertach moved quickly to assume complete power.
From his exile in Leinster, however, Diarmait fought back, supporting the Leinster king, Donnchad son of Domnall Remur (qv), in a failed encounter against Muirchertach at Ráith Étair near Dublin in 1087. The following year saw him attempting to make inroads closer to home: circuiting the coast of Cork with a naval fleet he plundered Cloyne and stole the relics of St Findbarr (qv) from Carrigaline. In this venture too, success was limited and he lost 200 of his men. He may have taken refuge in Connacht; if so, he was forced to flee north-eastwards to the territory of the Ulaid when Muirchertach was victorious in the west around 1092. One year later he accepted the inevitable, submitting to his brother in an elaborate ceremony in both Cashel and Lismore in the presence of Bishop Domnall Ua hÉnna (qv) and the nobles of Munster. The covenant agreed – with the relics of Ireland, including the bachall Ísu (St Patrick's crosier), being employed as pledges – may have involved Diarmait's re-affirmation as ruler of Waterford. In any event, in 1096 he appears as ‘Dermeth dux’, along with Muirchertach and others, as co-signatory to a letter from the people of Waterford to Anselm, archbishop of Canterbury, requesting that Máel-Ísu Ua hAinmire (qv) be consecrated their first bishop. Indeed, Anthony Candon speculates that Diarmait may have been governor of much of Munster from this time, controlling local affairs while his brother, by now established in the southern half of the country, sought to extend his authority in the north (‘Barefaced effrontery’, 9–10).
In truth, the chronicles are remarkably silent on Diarmait's activities in the first decade or so of the twelfth century. That he maintained a solid power base, however, can be inferred from the speed with which he reacted to Muirchertach's illness in 1114, seizing the kingship of Munster and expelling his brother from his Limerick capital to Killaloe. His triumph was short-lived: Muirchertach recovered sufficiently the following year to imprison his traitorous sibling. Yet the Munster king was indeed ailing and the conflict continued with Muirchertach being supported by his son and chosen successor, Domnall, and Diarmait having the backing of his three adult sons, Tairdelbach (qv) (d. 1167), Conchobar (qv) (d. 1142), and Tadc. Having captured Domnall, in 1116 Diarmait once more turned against his brother and forced him into retirement in Lismore. Yet, he proved no match for Munster's traditional enemies, both he and his son Tairdelbach being beaten by Connacht forces in subsequent encounters. In the end another enemy prevailed: Diarmait died, perhaps suddenly, in Cork in 1118. A victory of sorts was to come in death: a generous obit in the Annals of Ulster terms him ‘king of Munster and of the southern half of Ireland besides’. More significantly, it was his three sons, rather than the descendants of his brother, who took up the reins of power on Muirchertach's death in 1119.