Ua Néill, Áed (975–1004), son of Domnall and king of Ailech, belonged to the Cenél nÉogain dynasty. His father, Domnall Ua Néill (qv) (d. 980), was king of Tara and one of the most innovatory leaders of his time. His mother was Dirbail, daughter of Tadg Ua Conchobair, king of Connacht. Áed had at least two half-brothers, Muirchertach and Muiredach, and his aunt Dunlaith was mother of the Clann Cholmáin ruler Máel-Sechnaill (qv) son of Domnall, who took the kingship of Tara in 980. Áed was formally ordained to the kingship of Ailech in 993, when Muirecán abbot of Armagh went on a circuit of Tír nÉogain. On that occasion, he was conferred with kingly orders ‘in the presence of Patrick's (qv) community’.
In 999 Áed raided Uí Echach, east of the Bann, and in 1002 led a hosting to Tailtiu. That same year, Máel-Sechnaill offered to resign the kingship of Tara in his favour, in an effort to preserve the paramountcy of the Uí Néill dynasties and forestall the rise of Brian Bórama (qv). Áed, however, refused the offer, and the developing high-kingship of Ireland fell out of the hands of the Uí Néill. Seeking to enforce his lordship on the Ulaid, Áed defeated them heavily in 1004 at their inauguration site of Cráeb Tulcha (Crew Hill, parish of Glenavy, Co. Antrim); the fallen included the overking of Ulaid, Eochaid, son of Ardgal (qv), and many of his retainers. However, in the same battle – or in the immediate aftermath – Áed himself was slain; it appears that he was slain by members of his own dynasty, presumably dissidents. He was then 29 years of age and had reigned for ten years. He was succeeded in the kingship of Cenél nÉogain by his nephew Flaithbertach Ua Néill (qv).