Brian O’Neill (Irish: Brian Chatha an Dúna Ó Néill), the High King of Ireland from 1258 to 1260, is defeated and killed by the forces of Roger des Auters at the Battle of Down on May 14, 1260.

O’Neill is the son of Niall Roe O’Neill, and grandson of Áed in Macáem Tóinlesc. His wife is Nuala O’Connor (Ní Conchobair), a daughter of Rory O’Connor (Irish: Ruaidrí Ua Conchobair), the last High King of Ireland before the Norman invasion. Therefore, through his mother, he is descended from Brian Boru (Irish: Brian Bóramha).

In 1230, Hugh O’Neill (Irish: Aedh Ó Néill), king of Tyrone, dies and is succeeded by Donnell MacLaughlin. MacLaughlin, however, is removed in 1238 by the Justiciar of Ireland, Maurice FitzGerald, 2nd Lord of Offaly, and Hugh de Lacy, 1st Earl of Ulster, who install “the son of O’Neill”, presumed to have been Brian, and take the hostages of the Cenel Owen and Cenel Connell. However, it may have been Brian’s cousin Donnell, who afterwards is killed by MacLaughlin. After this, O’Neill claims the kingship of the O’Neill dynasty as well as Tyrone, possibly with the aid of Hugh de Lacy, 1st Earl of Ulster.

In revenge, O’Neill with the aid of Melaghlin O’Donnell, king of Tyrconnell, defeat MacLoughlin and ten of his closest kinsmen at the Battle of Camergi, somewhere within Tyrone north of Omagh, in 1241. This ends the long rivalry between the MacLoughlin’s and O’Neill’s, with the MacLoughlin’s afterwards excluded from the kingship of Tyrone and Ailech.

In 1244, Henry III of England sends letters to various Gaelic Irish lords, including O’Neill, requesting their aid in a military campaign against the Kingdom of Scotland. In the end the issue is sorted out diplomatically. Copies of the letter are also distributed to O’Neill’s sub-chiefs including his tánaiste, Hugh Boy O’Neill.

A consequence of this infighting between the rival factions of the Cenél Eoghain allows the Normans to advance deeper into Gaelic Ulster, however, in 1243 de Lacy dies. Thus the Earldom of Ulster reverts to the English Crown and is taken over by royal administrators. John FitzGeoffrey, the king’s chief governor in Ireland, erects a bridge across the River Bann and builds castles at Coleraine and Ballyroney in Iveagh. From here FitzGeoffrey is able to penetrate deeper into Tyrone.

Despite ending MacLoughlin aspirations to the kingship, O’Neill forms a marriage alliance with them, however, this results in a war with the O’Donnells of Tyrconnell. Subsequently in 1248 O’Neill backs the king of Tyrconnell, Rory O’Cannon (Irish: Ruaidri Ua Canannáin), against the claims of O’Donnell. O’Cannon had been set up in the kingship by FitzGerald, however, rather than backing him, enters Tyrconnell and removes him in favour of Gofraid O’Donnell.

O’Cannon, who had been expelled to Tyrone, and O’Neill once again lead their forces into Tyrconnell to confront O’Donnell, however they are defeated and O’Cannon is killed.

That same year, John FitzGeoffrey, who replaced FitzGerald as Justiciar in 1246, enters Tyrone and takes the submission and hostages of O’Neill. A resolution had been adopted at a meeting of the Cenel Owen that “since the power of the Foreigners was over the Gaeidhel of Erinn, to give hostages to the Foreigners, and to make peace with them, for the sake of their country.”

In 1249, the king of Connacht, Felim O’Connor, is given refuge from the Normans by O’Neill. In 1252, O’Neill and his brother give their submissions to the Justiciar of Ireland, who had marched to Armagh with a large force. A Rory O’Neill is given as hostage.

In 1253, as a sign of defiance against his vassal status with the Earldom of Ulster, O’Neill withholds his tribute to it and raids Iveagh, destroying the castle at Ballyroney. He also launches an offensive against the Normans in Leinster. That same year, the son of Maurice FitzGerald leads his forces into Tyrone to attack O’Neill, however he fails to take his submission or hostages and after battle suffers a heavy defeat at the hands of O’Neill.

In 1255, O’Neill makes a pact with Felim O’Connor’s son Hugh, whereby allowing Hugh free rein in the Kingdom of Breifne, he would aid O’Neill against the Normans of the earldom who are eroding his territory.

In 1257, the king of Tyrconnell, Gofraid O’Donnell, is mortally wounded in battle against the FitzGeralds and O’Neill uses this opportunity to try to exact Tyrconnell’s submission. As the Cenel Connell discuss what to do, Gofraid’s youngest brother, Donnell Óg, returns from fosterage and is conferred the chieftainship of Tyrconnell. He refuses to submit O’Neill stating the Scottish proverb “Every man should have his own world.”

FitzGerald in 1252 had built a castle at Caoluisce, on the banks of Lough Erne, near modern day Belleek, County Fermanagh, Northern Ireland, however, in 1258 it is the site where O’Neill, in the presence of his ally O’Connor, is inaugurated as “King of the Gael of Erin.” While he receives hostages from O’Connor and from O’Brien of Thomond, along with several other minor Kings from Meath and Munster, his claim is not recognised by those of the Irish closest to him including the other O’Neill factions, the O’Donnell’s of Tyrconnell, the MacMahon’s of Airgíalla, and the O’Rourkes of Breifne. The following year O’Donnell leads an attack into Tyrone.

In 1260, O’Neill, along with his O’Connor allies, launch an attack on the Normans of the Earldom of Ulster at Drumderg, near its capital at Downpatrick in modern County Down, Northern Ireland. The Normans levy the town, and with the aid of forces brought by Sir Roger des Auters, O’Neill and his allies are decisively defeated at the subsequent Battle of Down. The Annals of Inisfallen state that the forces recruited by the Normans consist mostly of native Irish and that the Normans play only a minor role.

In the battle, O’Neill is killed along with many other Irish nobles including over a dozen members of the O’Cahans. O’Neill’s head is cut off by the Normans and sent to King Henry III of England, a sign of how dangerous his coalition is believed to be.

After this battle, O’Neill becomes known in Irish as Brian Chatha an Dúna, meaning “Brian of the Battle of Down.”

After O’Neill’s death, the kingship of the Cenel Owen, and with it Tyrone, is taken by his cousin’s son, Hugh Boy O’Neill, ancestor of the Clandeboye O’Neill’s, who also has the support of the earldom of Ulster. Upon Hugh’s death in 1283 O’Neill’s son Donnell seizes the kingship, which until 1295 is highly contested between him and his second-cousin Niall Culanach O’Neill and Hugh Boy’s son Brian, until he wins outright control by killing his opponents.