In 489 was fought the battle of Cenn Losnada in Mag Fea in which he was slain. His wife Eithni Uathach ingen Cremthainn, called "the hateful", was also killed. She was a member of the Uí Cheinnselaig sept of the Laigin. The victors included Iollann mac Dunlaing, and Oilill, his brother of the Uí Dúnlainge sept of Laigin, and Eochaidh Guinech of the Uí Bairrche sept of Laigin, and, according to the Annals of Tigernach,  Muirchertach Mac Ercae, the Ui Neill king of Ailech. Óengus' head was given to Iollan. The reference to Muirchertach Mac Ercae as king of Ailech is unique to the Annals of Tigernach. A second entry in the Annals of Ulster with reference to the battle mentions Mac Ercae as victor, without reference to Ailech. According to Cormac McSparron  the reference to Muirchertach Mac Ercae as king of Ailech in the Annals of Tigernach probably arises from an insertion made after 913. The reason for its insertion may have been an attempt to push the expansion of the Uí Neill and their taking of Aileach farther back into antiquity than was the case.