[2] He would become king of Tyrone in 1260 after the death Brian at the battle of Down, however his brother Niall Culanach contested the kingship and sized it in 1261, holding it until Hugh Boy expelled him from it in 1262 and killed his main ally, Donnsléibe McCawell.
[2] In the early 1270s O'Neill along with some of his sub-chiefs, including O'Cahan, are mentioned in credits for accompanying the Justiciar of Ireland, James de Audley, in some expeditions.
[2][3] O'Neill however later that year received a lenient pardon for his part in the feud, even though it would drag on until 1276 when the de Mandeville's were defeated.
[3] At the battle of Desertcreat in 1281, the forces of O'Neill along with those of the new seneschal of Ulster, Thomas de Mandeville, decisively defeated and killed Donnell Óg O'Donnell and many of his sub-chiefs greatly curbing the power of his Cenél Conaill rivals.
His son Brian with the backing of Richard Óg de Burgh, 2nd Earl of Ulster, would replace Donnell as king of Tyrone in 1291.