O'Neill (Ó Néill), Brian (d. 1260), king of Tír Eógain and briefly high-king of Ireland (1258–60), was a son of Niall Ruadh O'Neill (d. 1222). During the reign of Brian's uncle Áed O'Neill (qv) from the late 1190s to 1230, the O'Neills became one of the dominant Irish dynasties in Ulster. On his death (1230), Áed was succeeded by his son Domnall Óc O'Neill (d. 1234). Domnall Óc was opposed by Domnall MacLoughlin (qv), his ambitious and powerful rival for the kingship of Tír Eógain. From his power base in Co. Londonderry, MacLoughlin began to expand his power. By 1232, MacLoughlin, with English help, was steadily expanding his influence into Tír Conaill, to O'Donnell chagrin. That year Domnall O'Donnell (qv) (d. 1241) retaliated, burning deep into Tír Eógain to Tulach Óc in O'Neill territory. This may suggest that Domnall Óc was allied to MacLoughlin. In 1234/5 MacLoughlin felt his hour had come and challenged Domnall for the kingship of Tír Eógain, killing him. This loss of the kingship was a great blow to the O'Neills. In early 1238 their luck changed when a follower of Hugh de Lacy (qv) (d. 1242), earl of Ulster, killed one of MacLoughlin's vassals, MacLoughlin's counter-attack resulting in the expulsion of de Lacy from the province. In MacLoughlin's greatest triumph lay Brian's opportunity. During harvest 1238 de Lacy and the justiciar Maurice FitzGerald (qv) (d. 1257) expelled MacLoughlin to Connacht. De Lacy gave the kingship of Tír Eógain to an O'Neill, who was perhaps Brian. In 1239 MacLoughlin returned to Ulster with an army and won a great victory over the O'Neills at Carnteel near Aughnacloy, then killed de Lacy's son along with twenty-eight knights. In 1241 Brian O'Neill formed an alliance with Maelsechlainn O'Donnell (qv), who feared the resurgent MacLoughlin. In early summer 1241 they attacked MacLoughlin. At the battle of Caimeirge (Cummery, in the townland of Erganagh north of Omagh), Brian and the O'Donnells annihilated MacLoughlin and his allies. Following his victory, Brian made a shrewd move by marrying Cecilia (d. 1250) of the fallen MacLoughlin dynasty.
For the next three years Brian confined himself to consolidating his rule over Tír Eógain. In 1245 he changed his policies and began to exert himself more forcibly in Ulster, authorising the assassination of Murchad O'Hanlon that year. Moreover, Brian was drawn into the politics of Connacht and Donegal because of the worrying advance of English power into the lands of the Irish. Clearly the O'Connors of Connacht and the O'Donnells of Donegal were becoming increasingly nervous, culminating in a major conflict with the English in 1247. That year the Cenél Eogain were present at Ballyshannon to defend Maelsechlainn O'Donnell from the forces of the former justiciar, Maurice FitzGerald (qv). FitzGerald was irresistible, defeating the Cenél Conaill and the Cenél Eogain and killing Máelsechlainn before installing Ruaidrí O'Cannon as king of Donegal. Later O'Cannon revolted, leading FitzGerald to switch his favour to Gofraid O'Donnell (qv). FitzGerald's attempt to impose him failed. But in 1248 FitzGerald did manage to expel O'Cannon to Tír Eógain, replacing him with Gofraid. Under Brian's protection, O'Cannon regrouped before returning to Donegal to challenge Gofraid. In spite of O'Neill assistance, O'Cannon was killed, setting the scene for a bitter rivalry between Gofraid and Brian. FitzGerald, however, was clearly worried by Brian's outright hostility to the advance of English authority. Determined to nip the O'Neill threat in the bud, FitzGerald invaded Tír Eógain and forced Brian to submit. This intervention, however, ultimately proved fruitless. Once FitzGerald departed, Brian put a fleet on Lough Erne, burning English settlements and castles the length of the lake. Before long Brian again turned his attention to Donegal, promoting Niall O'Cannon (d. 1250) against Gofraid and his English allies.
It is clear also that Brian was concerned by the developing crisis in Connacht. In 1249 a war broke out between the English and the Irish of west Leinster, contributing directly to unrest in Connacht. Then King Fedlimid O'Connor (qv) (d. 1265) of Connacht seemingly unleashed his son Áed (qv) (d. 1274) on the English. Before the English could strike back effectively, Fedlimid took refuge with Brian, and remained in Tír Eógain before returning to Connacht in 1250 to depose his renegade nephew Toirrdelbach O'Connor (d. 1266). Fedlimid's successes encouraged the English to recognise him again as king of Connacht. With Connacht temporarily pacified, FitzGerald, with Cathal O'Reilly (qv) (d. 1256) and his brother Cú Chonnacht (qv) (d. 1256), attacked Tír Eógain, forcing Brian to submit. This latest capitulation doomed Niall O'Cannon, Brian's protégé in Donegal, allowing FitzGerald to depose and kill him.
By 1252 Brian had recovered from these reverses and was promoting war against the English. Through a policy of coercion and alliance-building in Ulster, he again began to exert himself over the province. Indeed, his coercion of the O'Kanes prompted a rapid English reaction. The justiciar, John FitzGeoffrey (qv), was compelled to lead an army into Tír Eógain, forcing Brian to submit and to hand over his brother Ruaidrí as a hostage. Compounding O'Neill misery was the success of Gofraid O'Donnell's raids into Tír Eógain, culminating in his victory over Brian that year. In 1253, however, Brian enjoyed a year of previously unparalleled success, repelling FitzGerald's expedition into Tír Eógain. More success was to follow. That year he swept into east Ulster to exact a terrible revenge on the English there. According to the annals, he destroyed all the English townships and castles located on the eastern Ulster plains, including the castle at Drumore and the town of Dundalk in Louth. These successes perhaps can be in part attributed to his increasingly close links with the O'Connors, confirming his alliance with them in 1255. This came at a cost, resulting in Brian's three unsuccessful attacks on the O'Reillys, the mortal enemies of the O'Connors. However, Brian's mixed fortunes dogged him. During 1257 he tried to exploit the political crisis in Donegal caused by the mortal wounding of Gofraid O'Donnell in a battle with FitzGerald. In early 1258 Brian invaded Donegal and demanded Gofraid's submission. According to the dramatic account in the Four Masters’ annals, in true heroic style the dying Gofraid, laid on a bier, led his kinsmen to victory, routing Brian on the banks of Lough Swilly. That miserable defeat ironically heralded Brian's promotion of his right to the lapsed high-kingship. Later that year at Cáeluisce in Fir Manach, he was recognised as high-king of Ireland by Áed O'Connor of Connacht and Tadc O'Brien of Thomond. This was an event of immense political importance, heralding the birth of an alliance between the great Irish houses of O'Neill, O'Connor, and O'Brien. Around the time of Áed O'Connor's marriage to Mac Sorley's daughter in 1259, Brian may also have taken a new wife, Julia, daughter of a Scottish lord. Brian's elevation and the emergence of these alliances caused the English government grave concern. About this time Brian judged that the time was ripe to give a practical demonstration of the power of his high-kingship, demanding the submission of Domnall Óc O'Donnell (qv). In 1259 O'Donnell gave his answer, linking up with Áed Buide O'Neill (qv) to burn Tír Eógain and Monaghan. Another blow was the death of Tadc O'Brien (1259), depriving Brian of a key ally in the south. Although O'Brien was dead, Brian and Áed O'Connor showed that the alliance of Cáeluisce was still workable. During the year they renewed it at Lough Erne and installed Domnall O'Rourke (qv) (d. 1260) as king of Breifne. The relative success of this alliance continued. In 1260 Brian summoned Áed to join him on campaign against the English of Ulster. Áed O'Connor brought a large army of his Connacht vassals, along with a sizeable contingent of O'Melaghlins from Meath, to link up with Brian. Outside Down (16 May 1260) they were heavily defeated by the English of Ulster possibly assisted by Áed Buide O'Neill. While Áed O'Connor managed to escape from the slaughter, Brian was beheaded and his head was sent to England.