Brian Ua Néill, also known as Brian O'Neill, was the High King of Ireland from 1258 to 1260. His status as High King is disputed by some, as the forces of the O'Briens of Thomond failed to support him in the Battle of Druim Dearg in 1260, which contributed to him being killed. However, they had both submitted by treaty to him and given hostages to him prior to that.
Due to the increasing pressure of the Norman colonists, under the Lordship of Ireland, revolts began to flare across Ireland. Tadhg O'Brien, son of the King of Thomond defeated the colonists in 1257 and plundered their lands. Hugh de Lacy's death in 1243 meant that the Earldom of Ulster was still in a period of lax administration. In 1255, Brian (who at this time was King of Tir Eoghain) seized this opportunity to raid the colonists' lands across the River Bann into Ulaid and destroy any towns and castles that he encountered. Meanwhile, Aodh O'Connor, the son of the King of Connacht, expanded his territory by conquering the neighbouring Kingdom of Breifne in 1256 (with backing from Brian).
Brian, Tadhg, and Aodh formed an alliance, and in 1258 they met near a ruined Geraldine castle at Belleek on the Erne. Here Brian was confirmed as High King of Ireland and Aodh's overlordship of Breifne was accepted. However their success was short-lived. Tadhg died the following year, and Brian and Aodh had lost a valuable ally.
In 1260 Brian and Aodh assembled an army and attacked the Norman colonists near Downpatrick. Expecting an attack, the Normans had also raised an army, consisting mostly of levies from the native Irish who were either allied to them or under their rule at the time (i.e. the Irish of Leinster, Munster, some of Connacht and Meath). At the Battle of Druim-dearg, Brian and Aodh were heavily defeated, and Brian was killed along with many other important Irish leaders (including a number of O'Cahan chiefs). The Annals of Inisfallen state that the forces recruited by the Normans consisted mostly of native Irish and that the Normans played only a minor role. Brian's head was cut off by the Normans and sent to King Henry III of England.