Tuathal Máelgarb (‘rough-cropped’) (d. 544), son of Cormac Cáech and king of Tara, belonged – it is claimed – to Cenél Cairpri, a dynasty that became associated with Uí Néill. Nothing is recorded of his father Cormac, except his place in the dynastic pedigree. Tuathal's mother is said to have been Commain, daughter of Dallbrónach. According to the genealogists, his grandfather Cairpre (qv) was a son of Niall Noígiallach (qv); certainly, his line was closely connected with other dynasties of the Uí Néill confederation in the later sixth and early seventh centuries.
Tuathal's status as king of Tara is recognised by his apparent inclusion in the regnal list ‘Baile Chuinn’ under the kenning of ‘Óengarb’ (uniquely rough). His predecessor is named in the same source as Mac Ercae, merged in later tradition with a Cenél nÉogain figure to create the compound persona Muirchertach/ Mac Ercae (qv). Tuathal's accession to the kingship is placed at 534. Shortly afterwards, he inflicted a defeat on the Cianachta of Brega at Luachair (Logher, Co. Meath). Little is known of his subsequent reign, which (according to medieval regnal lists) lasted eleven years. Later tradition claims that he drove his rival, Diarmait (qv) son of Cerball, into exile.
In 544 Tuathal was assassinated at Grellach Elti by a certain Máel-mórdai son of Airgetán. Allegedly, Máel-mórdai was an uterine brother of the exiled Diarmait; in any event, the latter did profit from Tuathal's death and eventually secured the kingship of Tara. For several generations Tuathal's descendants remained powerful in the midlands and in north Connacht, but by the late seventh century the dynasty was in decline.