Brian Ua Néill, also known as Brian O'Neill, was the High King of Ireland from 1238 to 1260.He came to power with the help of Malachy O’Donnell who helped defeat his kinsmen from the tribe of Ui’Neill Donal McLaughlin for control of the ancient territory of Tyrone. This was an ongoing battle between the two where one would dislodge the other; this pattern continued over some years until Brian finally overcame Donal and his supporters in 1241 at the battle of Caimerge.

The battle site is probably just outside of the town of present day Maghera at the townland of Caimerge. It is from this point that Brian is known as the King of Tir Eoghain. Another major factor in Brian’s development as the O’Neill came from Hugh de Lacy's death in 1243 which meant that the Earldom of Ulster was still in a period of lax administration and Brian wasn’t one to miss an opportunity.

In 1244 King Henry III King of England wrote the following letter Brian when he was about to march against the Scots and their King.

“The King to Brian O’Neil, King of Kinelun, health whereas the King of Scotland has provoked us by injuries, and that we have prepared to rise against him in revenge for his many transgressions, unless he makes reparation, confiding in your love that you will not deny us your help, in this our expedition; we request you to come along with our justicary of Ireland ,and other liege men of Ireland, who are soon to come to parts of Scotland to distress our enemies there; and that you would be pleased to give us such and so powerful a succour, appearing persoanlly at the head of your brave forces, that you may in any distress apply to us for succour with full confidence. And by granting this our prayer whatever favor you ask, we most willingly shall acknowledge ourselves bound to grant with special act thanksgiving. Witness the King, at Stanford, 7th day of July