Ua Briain, Domnall (d. 1115) was a son of Tadc Ua Briain (d. 1086) son of Tairdelbach (qv) (d. 1086). His mother, Mór, was the daughter of Echmarcach (qv) son of Ragnall of the Waterford vikings and sometime king of Man, who had retired on pilgrimage to Rome in 1064 with his brother-in-law, Domnall's great-granduncle, Donnchad (qv) (d. 1064) son of Brian Bórama (qv). Along with his brothers, Donnchad and Amlaíb, Domnall was involved in the succession dispute following the unexpected death of his father in 1086. A sea expedition undertaken by them to Man the following year, with the assistance of the son of the king of the Ulaid, may have been designed to undermine the authority of their uncle, Muirchertach Ua Briain (qv), who had seized the kingship and banished his brother Diarmait (qv) on Tadc's death. As Seán Duffy has noted, since Muirchertach was no longer in control of Dublin at this time, any attack on Man – where successive powers in Dublin had long since sought to wield authority – would be construed as a threat to the would-be ruler (‘Irishmen and Islemen’, 105). In 1091, by which time Muirchertach had Dublin once more firmly in his grip, he came to terms with his nephews, but the peace was not to last. By now, however, the elder Ua Briain faced a greater threat in the east in the person of Gofraid Meránach (qv), king of Man, who had set his sights on Dublin. Muirchertach succeeded in expelling him in 1094 and he died in Islay the following year.
What happened immediately following Gofraid's death is not entirely clear. According to a Manx source, ‘Cronica regum Mannie et insularum’, the noblemen of the Isles approached Muirchertach, ‘requesting that he send some worthy man of royal stock to act as regent until Gofraid's son Amlaíb (Óláfr) came of age’ (Duffy, ‘Irishmen and Islesmen’, 109). Domnall Ua Briain was then dispatched, but after a tyrannical rule lasting three years he was forced to flee to Ireland. Although Irish chronicles relate that Domnall's brother, Amlaíb, was killed in Man during this period, in 1096, they place what is described as Domnall's forcible seizure of the kingship of the Isles in 1111. If the evidence pertaining to the earlier tenure is in fact reliable, we must assume that Muirchertach deliberately placed a dangerous opponent in a position of authority, perhaps as an attempt to assuage and control him; Domnall's dynastic connections with the territory may also have made him an obvious choice as ruler. If so, the younger Ua Briain did not profess unswerving loyalty to his uncle as a result, for Muirchertach was forced to imprison him in Dublin in 1107. Against this background, it seems likely that the attack on the Isles four years later was undertaken without his uncle's support and, as Duffy notes, Muirchertach's three-month sojourn in Dublin in that year may have been to enable him to keep a close eye on neighbouring events (ibid., 114). In any event, Domnall's success was short-lived: Gofraid Meránach's son, Amlaíb, became king the following year.
Not surprisingly, Domnall was actively engaged in plotting against Muirchertach after his uncle fell ill in 1114. He acquired the support of the latter's arch-opponent, Tairdelbach Ua Conchobair (qv), for a time and as a result seized control of Tuadmumu (Thomond) the following year. This opportunistic alliance was not to last and Domnall was in fact killed by the Connachta later that year, his death marking the end of any pretensions this branch of the family had to the kingship.