Ua Briain, Tairdelbach (1009–86) was the son of Tadc (qv) son of Brian (d. 1023) and a grandson of Brian Bórama (qv). His mother was Mór, daughter of Gilla-Brigte Ua Maíl Muaid, king of Cenél Fiachach. Among his wives was Gormlaith, daughter of Ua Fócarta, king of Southern Éile, who predeceased him by a decade. In 1023 his father was treacherously slain at the instigation of the reigning king, Tairdelbach's uncle, Donnchad (qv) (d.1064) son of Brian. Not surprisingly, therefore, Tairdelbach was involved from an early age in attempts to undermine Donnchad's authority. He was aided in this by his foster-father, Diarmait (qv) son of Máel na Mbó (qv), who was in fact the son-in-law of Tairdelbach's uncle. Notwithstanding this relationship, the Leinster king assiduously cultivated Tairdelbach as his protégé. In 1054, bolstered by the support of Áed Ua Conchobair (qv), king of Connacht, the pair launched a coordinated attack on Donnchad's north Munster base and on his fortress at Dún Trí Liac; Tairdelbach was harrying in Clare again the following year. Two years later, Diarmait sent his Dublin allies to plunder Scattery Island, and the Norsemen were also involved in an assault on Limerick led by Tairdelbach and his Leinster ally in 1058. In the encounter, termed ‘the battle of Sliab Crot’ (AU), Donnchad's power was effectively crushed though he continued to rule for a further five years. It was only in 1063 after Limerick had been burned once again by his nephew in alliance with the Leinster kingmaker that Donnchad fled to Rome: the reign of Tairdelbach Ua Briain could now begin.
In an effort to consolidate his authority in his home base, Tairdelbach initially led a number of raids into Munster territories. Corco Duibne and Éoganacht were attacked in 1064, while the king of the Déssi was captured in 1067. Simultaneously, his attention also turned further afield: he joined Diarmait son of Máel na mBó in the latter's support of Áed Ua Ruairc (qv) in the same year and took part in the hosting to Connacht on which Áed Ua Conchobair was slain. The following year his continuing allegiance to the Leinster king was marked by a visit to Diarmait's territory, in the course of which he was presented with many valuable gifts, including his grandfather's sword and the standard of Edward the Confessor. Diarmait may not have been too pleased, however, when Tairdelbach accepted the submission of the king of Osraige in 1070. Be that as it may, it was to the Munster king he turned for support when the sons of his brother Domnall Remur (qv) (slain 1041) sought to supplant him the following year. Tairdelbach himself may have been involved in an attempt to overthrow his erstwhile ally in 1072; in any event he was fighting alongside Conchobar Ua Máelshechlainn (qv) against Diarmait in a battle in Mide in which the Leinster king was slain.
Tairdelbach was now in a position to come into his own. Intent on establishing his authority in Diarmait's home base, he immediately marched into Leinster and Osraige, taking their hostages, the Norsemen granting him the kingship of Dublin on the same occasion. When Ua Máelshechlainn of Mide submitted to him later that year, his control over the southern half of the country was no longer in doubt. Yet he did not become complacent: he attacked Mide again in 1073 and two years later enforced his rule in Dublin by banishing the current king, Gofraid grandson of Ragnall. Donncha Ó Corráin suggests that he appointed Domnall, king of Áth Cliath and grandson of Diarmait son of Máel na mBó, in his place, in an attempt to keep Leinster divided between Diarmait's kinsmen and those of his brother, Domnall Remur, who were ruling Uí Chennselaig (Ireland before the Normans, 138). If so, Domnall's death later that year necessitated a change of plan and Tairdelbach granted the kingship to his able son, Muirchertach (qv) (d. 1119), in his stead. The Leinstermen were frequently reminded of his overall control, however: in 1077, he marched into Uí Chennselaig territory and took the king, Donnchad son of Domnall Remur, prisoner.
Connacht leaders suffered the same fate, Ruaidrí na Saide Buide Ua Conchobair (qv) having been taken captive by Tairdelbach the previous year. In the west too, the Munster king was fortunate in that dynastic strife had weakened the territory. Moreover, as Ó Corráin has noted, Tairdelbach made every effort to keep Connacht divided, expelling Ruaidrí Ua Conchobair after he had seized the kingship in 1079 (ibid., 139). Yet he proved powerless to prevent the growth in power of Ua Conchobhair's main rival, Ua Ruairc, whose attention was directed eastwards towards Mide, as well as westwards. Tairdelbach was not slow to react: after a hosting into Mide and Leinster in 1080, the king of Mide submitted to him in Dublin, as well as in Limerick. The Munstermen were in Mide again in 1084 when they were attacked by Donnchad Ua Ruairc who was defeated by Tairdelbach's son, Muirchertach, in the battle of Móin Cruinneóige, the enemy's head being brought back to Limerick in triumph. In Ua Ruairc's army was Cennétig Ua Briain, a grandson of Tairdelbach's despised uncle, Donnchad, who had been made ruler of Gailenga by the Connacht ruler. He too was slain at the hands of his estranged kinsmen.
Elsewhere, Cennétig is described as king of Telach Óc in which position he succeeded his brother, Conchobar, who was killed by the Cenél mBinnig in 1078. His patron for this position was more likely to have been Domnall Ua Lochlainn (qv), whose wife, Bébinn, was Conchobar Ua Briain's niece. Domnall himself became head of Cenél nÉogain in 1083 and it was only under his rule that the north became a force with which to be reckoned. For the greater part of Tairdelbach's rule, therefore, he did not need to concern himself unduly with northern affairs. He made one unsuccessful hosting into Airgialla in 1075; his authority was acknowledged, however, by successive rulers of the Ulaid, who travelled south to submit to him in 1078 and again two years later.
Tairdelbach's standing and influence were similarly recognised further afield, as is evident in letters sent to him by successive archbishops of Canterbury, Lanfranc and Anselm, as well as by Pope Gregory VII. In the eyes of these foreign ecclesiastics, it was he who was ‘king of Ireland’ and it was to him they must turn to garner support for ecclesiastical reform. That the Munster ruler was sympathetic can be deduced from his involvement in the appointment of Patrick (qv) (d. 1084) and Donngus Ua hAingliu (qv) as bishops of Dublin, both of whose connections were with English monasteries, Worcester and Canterbury respectively. His control of Dublin may also have led him to cast his eyes occasionally across the Irish Sea. In 1073, one year after he had secured the kingship of the city, two unidentified Uí Briain were killed in the Isle of Man. What, if any, their connection with Tairdelbach was is unknown; as Seán Duffy has remarked, however, ‘it can hardly be doubted that their presence in Man . . . was part and parcel of Munster's attempts to control the city and its insular possessions’ (‘Irishmen and Islesmen’, 102). His fame was such that six years later, a number of Jews arrived at his court bearing gifts; whatever the purpose of their visit, they were not permitted to remain, being sent back again over the sea.
Notwithstanding these occasional outward glances, it is on his overview and control of the insular situation that Tairdelbach's reputation rests. Moreover, his skill and sagacity were such that that reputation is secure. Determined and decisive, he quickly regained for Munster the pre-eminent position it had enjoyed during his grandfather's rule, but had subsequently lost during the reign of his uncle, Donnchad. After his assumption of widespread control on the death of his one-time patron, Diarmait son of Máel na mBó, repeated interventions both of a military nature and in the political affairs of his opponents meant that his control of the southern half of the country was never in doubt. Dublin, too, remained firmly within his grip. A divided north gave him little cause for concern; such was his influence, however, that far-removed Ulaid leaders also submitted to his rule. Accordingly, he is granted the title rí Érenn (king of Ireland) even in a northern source, the Annals of Ulster, on his death from illness in 1086. Three of his sons, Muirchertach, Tadc, and Diarmait (qv) (d.1118), all of whom were politically active during his lifetime, succeeded him.