Ua Briain, Tairdelbach (d. 1167) was a son of Diarmait Ua Briain (qv) (d. 1118) and Mór, daughter of Ruaidrí na Saide Buide Ua Conchobair (qv). He proved a loyal supporter of his father in the latter's struggle with his brother, Muirchertach (qv) (d. 1119), in the final years of Muirchertach's reign. In 1117 he was defeated in Ormond by an army of Connacht men fighting alongside one of his uncle's key allies, Brian son of Murchad Ua Briain. He exacted sweet revenge two years later, when, along with his brothers, Conchobar (qv) (d. 1142) and Tadc, he assumed control on Muirchertach's death. Their power was significantly curtailed, however, by Tairdelbach Ua Conchobar (qv) (d. 1156), king of Connacht, who divided Munster between the Uí Briain brothers, who were confined to the northern territory of Tuadmumu (Thomond), and their neighbours, Meic Carthaig, who were granted Desmumu (Desmond) in the south. Further interference followed in 1122 when, in an attempt to cause dissension among Uí Briain, Ua Conchobair captured Tairdelbach and granted the kingship to his brother, Tadc. Tairdelbach had been released by 1125 at the latest, since he attacked Limerick in that year, in which Cormac Mac Carthaig (qv) took the kingship of the city.
Common cause against a mutual enemy, Ua Conchobair, was to bring the Munster rivals together; and when Cormac Mac Carthaig was deposed two years later, it was Tairdelbach and his brother Conchobar who brought him out of exile in Lismore and made him consensus king of Munster. Their fruitful alliance was to last a further seven years during which Tairdelbach acted as second-in-command to his increasingly powerful brother, Conchobar. Inevitably, growing confidence led Uí Briain to eschew their erstwhile partners, especially since the fortunes of the Connacht ruler suffered a temporary downturn in the mid 1130s. When hostilities with Meic Carthaig resumed about 1134, Tairdelbach Ua Briain assumed an active role, being at his brother's side during an encounter at Waterford in 1137. Indeed Conchobar owed his assumption of the kingship of Munster one year later directly to his brother since, according to various annalistic accounts, Cormac Mac Carthaig was treacherously slain in his own home either by Tairdelbach himself or his accomplice.
Little is known of Tairdelbach's activities during Conchobar's relatively short reign. There is no reason to suppose, however, that he did not remain loyal to his brother, whose kingship he assumed on Conchobar's unexpected death in 1142. His reign began reasonably well and he proved himself to be an active opponent of Tairdelbach Ua Conchobair, reacting in particular against the latter's forcible division of Mide. In 1144 at Terryglass, in the presence of laymen and clerics, the two leaders negotiated a truce ‘for as long as they should live’ (oired nobedis ’na mbethaidh) (Ann. Tig.), but both sides were at war again the following year. In that encounter and again in 1146, the Connacht ruler was victorious; Tairdelbach emerged triumphant two years later, demolishing Ua Conchobair's fort at Galway in the process. The Connachtmen exacted revenge in 1150, plundering Munster in Tairdelbach's absence. By now, however, Ua Briain had turned his attention to the northern ruler, Muirchertach Mac Lochlainn (qv), who seemed intent on extending his authority southwards. In a show of power, he attacked Mide in that year and continued into Dublin, where the foreigners submitted to him. Mac Lochlainn marched against him; but with the men of Dublin as peace-brokers, the two rulers came to terms.
This was to be the high point of Tairdelbach's career. Rival elements within his own dynasty, long since active, and a resurgence in the power of Meic Carthaig, effectively put an end to his ambitions. The year 1151 in particular proved catastrophic. Deposed by his son, Muirchertach, he was initially restored to the kingship with the aid of his brother, Tadc, whom Tairdelbach had imprisoned in 1147. Tadc evidently nurtured ambitions of his own, however, and acquired the ready support of Tairdelbach Ua Conchobair. Taking advantage of Ua Briain's misfortunes, Meic Carthaig too revived under a new and vibrant leader, Diarmait, the first of note since Donnchad Mac Carthaig had died as Tairdelbach's prisoner in 1144, and similarly turned to Ua Conchobair. The Connacht king obliged, marching into Munster to divide it between Mac Carthaig and the besieged Tairdelbach. He returned some months later, supported by Diarmait Mac Murchada (qv) of Leinster, Tigernán Ua Ruairc (qv) of Bréifne, and Murchad Ua Máelshechlainn (qv) of Mide. Ua Briain stood not a chance, suffering a calamitous defeat at Móin Mór near Fermoy, in which his losses numbered some 3,000 men; he was forced to flee from Munster.
Although Tairdelbach was restored to power in 1153 by Muirchertach Mac Lochlainn, who was anxious to curtail the advances of the Connacht king, he never truly recovered. Three years later he submitted to Ua Conchobair; on the latter's death later that year, he offered his son and successor, Ruaidrí Ua Conchobair (qv) (d. 1198), renewed submission in the form of twelve hostages. Perturbed by this turn of events, Mac Lochlainn deposed Ua Briain in 1157 but he was reinstated the following year. Loyalty to Ruaidrí Ua Conchobair was demonstrated again in 1160 and 1161 by a further exchange of hostages. It proved to have little effect in 1165, however, when Tairdelbach lost the kingship to his son, Muirchertach, whose supporters included the Connacht king. And while he nominally recovered power the following year, he is notably absent from a hosting undertaken by Ruaidrí Ua Conchobair, along with Tairdelbach's son, Muirchertach, Diarmait Mac Carthaig, and a host of other nobles in 1167. Despite being termed rí Muman ocus Lethi Moga (king of Munster and of the southern half of Ireland) on his death later that year, in reality his power had long since disappeared.