Óengus (d. 492?), son of Nad-fraích and putatively king of Cashel, was an important ancestor figure of the dynasty of Éoganacht Chaisil. According to the genealogists, Óengus and his brother Ailill were grandsons of Conall Corc (qv), the supposed founder of Cashel. Likewise, from genealogical sources it appears that his (principal) wife was Eithne Uathach, daughter of Crimthann son of Énnae Cennselach (qv), and that she was the mother of his sons Bressal, Senach, and Áed Cáech. Several other sons (apparently by other women) are expressly named; they include Eochaid, Dub-gilcaich, Fedelmid, Caírthenn, and Nath-Í, along with a daughter Uchdelb who is said to have married Ailill Molt (qv). Altogether, Óengus is credited with an improbable total of twenty-four sons and twenty-four daughters.
Óengus's reign at Cashel, which according to the regnal lists was thirty-six years, is synchronised with the mission of St Patrick (qv). Middle Irish Patrician hagiography claims that the saint encountered Óengus in Mag Femin, a plain in south-east Co. Tipperary. It relates how Patrick inadvertently thrust his crosier through the king's foot while greeting him; Óengus made no comment, thinking it was part of the Christian ceremony. It was believed that he became a zealous Christian, bestowing twelve of his sons and twelve of his daughters on the church. Later tradition credits him with having granted a site in the Aran Islands to St Énnae (qv).
In the political sphere, tradition has it that Óengus clashed with Duí Iarlaithe (supposedly his cousin), an ancestor of Éoganacht Locha Léin. The tale ‘The expulsion of the Déisi’ claims that he granted lands, which straddled south Co. Tipperary and north Waterford, to exiles from Meath who had helped him drive back the Osraige. The slaying of Óengus by Mac Ercae (along with his wife Eithne, as some versions have it) at the battle of Cenn Losnada in Mag Fea (near Old Leighlin, Co. Carlow) is assigned to the years 490 or 492. According to the regnal lists, his sons Eochaid, Fedelmid, and Dub-gilcaich succeeded to the kingship in turn. They were claimed as ancestors of the later Éoganacht Chaisil and Éoganacht Glendamnach kings; it is stated that the descendants of these three included, respectively, Cathal (qv) son of Finguine, Fedelmid (qv) son of Crimthann, and Fáilbe Flann (qv) son of Áed Dub. Their half-brother Bressal is reputedly the ancestor of Cormac (qv) son of Cuilennán.