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. Around this time a feud would arise within the Earldom between the de Mandeville's and the seneschal of Ulster, William FitzWarin. O'Neill took the side of Sir Henry and Sir Robert de Mandeville and is noted in 1273 along with his O'Cahan vassals as having burned five towns before FitzWarin routed them. Niall Culanach, now king of Inishowen (a sub-kingdom within Tyrone) saw an opportunity following this and offered his assistance to King Edward I to destroy O'Neill and alleged that he had some protection from the authorities in Dublin. 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.
