[3] In revenge, O'Neill with the aid of Melaghlin O'Donnell, king of Tyrconnell, defeated MacLoughlin and ten of his closest kinsmen at the battle of Camergi, somewhere within Tyrone north of Omagh, in 1241.
In 1244, Henry III of England sent letters to various Gaelic Irish lords, including O'Neill, requesting their aid in a military campaign against the Kingdom of Scotland.
[7] A consequence of this infighting between the rival factions of the Cenél Eoghain allowed the Normans to advance deeper into Gaelic Ulster, however in 1243 de Lacy died.
[4][5] John FitzGeoffrey, the king's chief governor in Ireland, erected a bridge across the River Bann and built castles at Coleraine and Ballyroney in Iveagh.
[12] FitzGerald in 1252 had built a castle at Caoluisce, on the banks of Lough Erne, near modern-day Belleek, County Fermanagh, however in 1258 it was the site where O'Neill, in the presence of his ally O'Connor, was inaugurated as "King of the Gael of Erin".
[4][5] Whilst he received hostages from O'Connor and from O'Brien of Thomond, along with several other minor Kings from Meath and Munster,[7] his claim was not recognised by those of the Irish closest to him including the other O'Neill factions, the O'Donnells of Tyrconnell, the MacMahons of Airgíalla, and the O'Rourkes of Breifne.
[12] In 1260 O'Neill along with his O'Connor allies, launched an attack on the Normans of the Earldom of Ulster at Drumderg, near its capital at Downpatrick in modern County Down, Northern Ireland.
[5] The Normans levied the town, and with the aid of forces brought by Sir Roger des Auters, O'Neill and his allies were decisively defeated at the subsequent battle of Down.
[16] Giolla Brighde Mac Con Midhe wrote the lament Aoidhe mo chroidhe ceann Briain (Brian's head is the care of my heart).