Tuathal (d. 958), son of Augaire and overking of Leinster, belonged to the Uí Muiredaig lineage of Uí Dúnlainge. His father, Augaire son of Ailill, provincial king in his time, had been killed by the Norsemen (917) at the battle of Cenn Fuait (probably Glynn, near the River Barrow, parish of St Mullins, Co. Carlow), which facilitated the restoration of the kingdom of Dublin. Although there is no record of Tuathal's wife, he had at least two sons, Augaire and Dúnlaing (qv). He was probably too young to have taken an active part in the battle of Belach Mugna (barony of Idrone, Co. Carlow) against the Munstermen (908), much less to have been king of Uí Muiredaig at the time, as claimed by the Fragmentary Annals (§423); this claim relates to his father. However, he did establish Uí Muiredaig – based in Co. Kildare – as a political force in Leinster, at a time when Norse Dublin exercised considerable sway in the north of the province. He also laid the foundations for his dynasty's later dominance over several key ecclesiastical sites, including Glendalough.
Tuathal had apparently achieved political power by the 930s, and dealt ruthlessly with any opponents. He especially directed his energies against the dynasty of Uí Chennselaig, which dominated southern Leinster, and the lineage of Uí Fháeláin, their closest rivals within Uí Dúnlainge. In 935 he slew Bruatar son of Dubgilla, king of Uí Chennselaig, and two years later he wounded Murchad son of Finn, whose uncle, Bran was already the most powerful magnate of Uí Fháeláin. It appears that at this stage Tuathal's dynasty was still tributary to Dublin; in 938, when Donnchad Donn (qv) son of Flann Sinna (qv) king of Tara took action against the Norsemen, he pillaged from Áth Cliath to Áth Truisten (near Mullaghmast, Athy, Co. Kildare). It seems likely that in the course of the following decade Tuathal profited from a series of catastrophes that befell the Hiberno-Scandinavian kingdom: the sacking of Dublin in 944 by the new king of Tara, Congalach Cnogba (qv) son of Máel-mithig, and two severe defeats (947–8) at the hands of the same ruler. No doubt his cause was further assisted when in 947 Bran son of Máel-mórda, now nominal overking of Leinster, was killed on a raid into Osraige.
Tuathal's attainment of overkingship, however, may not have proceeded smoothly: he is accorded a reign of only eight years in the Leinster king-list, which seems to date his accession to 950. By the early years of that decade, he was sufficiently well established to take the offensive against the neighbouring kingdom of Osraige. In 952 he raided two sub-kingdoms of Osraige: Loíchis and Uí Fhairchelláin. Still, it is noteworthy that, a year later, he had the support of the king of Dublin, Amlaíb (Óláfr) Cuarán (qv), in attacking Inis Ulad. Despite its location near Donard, Co. Wicklow, this site may have been linked with Uí Chennselaig interests – it shared an abbot with Taghmon (Co. Wexford). It is perhaps significant that Inis Ulad was later subject to Glendalough. In 957 Tuathal invaded Uí Chennselaig and defeated its king in battle.
He died the following year. Although his immediate successor in the overkingship was Cellach son of Fáelán of Uí Dúnchada, his sons later attained the provincial dignity. Through these, Tuathal became ancestor of two family lines: Augaire, who fell in battle against the Norse of Dublin at Bithlann (978), had a son, Lorcán, who gave his name to the line of Ua Lorcáin (O'Larkin), while the descendants of Dúnlaing (d. at Glendalough, 1014) were known as Uí Thuathail (O'Toole).