O'Brien (Ó Briain), Brian Bán (d. 1350), lord of Thomond, was second son of Domnall son of Brian Ruaid O'Brien (qv). A powerful warlord he played an important role in Irish politics between 1320 and 1350. He first comes to notice as the loyal lieutenant of his brother Donnchad, king of Thomond. Together the brothers effectively opposed the designs of their cousin and bitter rival Muirchertach O'Brien (qv) on the kingship of Thomond. After the retreat of the Scots from Munster (April–May 1317) the brothers, despite successes against Muirchertach, were forced to agree a truce. In June 1317 Muirchertach and his overlord Richard de Clare (qv) went to Dublin to attend a parliament. During their absence from Thomond, Muirchertach's brother Diarmait decided to bring Donnchad and Brian Bán to battle. Near the abbey of Corcumroe (15/17 August 1317) he completely defeated Donnchad, killing him. However, Brian Bán and a small number of horsemen hacked their way out of the slaughter and fled northwards across the Burren to Connacht. When de Clare returned, he was appalled by the dominance of Muirchertach's supporters; to counter this, he endorsed the tánaiste Mathgamain O'Brien as his favourite and recalled Brian Bán from Connacht. However, Muirchertach, having fought so hard for the Thomond kingship, was in no mood to share his prize. Conflict was inevitable. De Clare resolved to teach him a lesson, but he was woefully unprepared for an offensive. With Brian Bán in his train, de Clare set out to corner and kill Muirchertach. But an ambush by O'Brien's vassal, Conchobar O'Dea, at the ford of Dysert O'Dea, cost the headstrong de Clare his life (10 May 1318). Despite de Clare's death, Brian Bán rallied his army and encircled the O'Deas, hacking them down. Victory was snatched from his grasp by the arrival of Muirchertach's forces, tilting the balance against him. A fearsome slaughter followed, forcing Brian Bán and the survivors to flee to Bunratty. But Muirchertach kept on the pressure by the intensity of his pursuit. At Bunratty, Brian Bán and the survivors clambered aboard galleys as Muirchertach closed for the kill. Once on the other side of the Shannon, Brian Bán gathered his remaining followers and began to reconquer the former O'Brien lands in Ormond from the de Burghs and the Butlers.
By the early 1320s Brian Bán had allied with Maurice fitz Thomas FitzGerald (qv), later earl of Desmond. During 1321 Maurice fitz Thomas was given possession of the de Clare lands in Limerick and Thomond, leading him into conflict with Muirchertach. Indeed, this alliance greatly bolstered Brian Bán's regional position and may have contributed to his defeat of the colonists in 1322. This notorious relationship worked two ways, and Brian Bán loyally supported fitz Thomas's endless ambitions, attending a meeting in Kilkenny in July 1326. There it was alleged that the earls of Kildare (qv) and Louth (qv), and the future earls of Ormond (qv) and Desmond, together with the bishop of Ossory and Brian Bán, agreed to rebel against the king, assume control of Ireland, and elect and crown Maurice king of Ireland. However, nothing came of this, but the pair continued their activities throughout much of Munster. By 1328 Muirchertach and the de Burghs decided to rid themselves of the troublesome Brian; but at Thurles (14 July 1328) he destroyed them in a pitched battle, giving Muirchertach and the MacNamaras similar attention later in the year.
The emergence of serious warfare in Leinster in 1329 led to unusual roles for both fitz Thomas (now earl of Desmond) and Brian Bán. In January 1330 the justiciar, John Darcy (qv), asked them to restore the king's peace in the Leinster marches. In this surprising position, the pair did not disappoint. With a large army they quelled the disturbances of the O'Nolans, O'Mores, and O'Dempseys. By May 1330, however, Brian Bán had returned to his usual form by killing James de Beaufo, sheriff of Limerick, and in July he fought the army of Roger Outlaw (qv) to a standstill. In 1332 fitz Thomas's alleged designs on the Irish crown had reemerged, leading to a pact with Brian Bán, MacNamara, William de Bermingham (qv), and Walter de Burgh (qv). By 1335 fitz Thomas had tired of his royal ambitions and now sought to become respectable. Unsurprisingly this led to a rift with Brian Bán; such was the bitterness that fitz Thomas led an expedition against his old friend late in the year. Brian Bán was in no mood to temporise, and burned the town of Tipperary in June 1336. However, the shift in the political landscape forced him to review his position. Realising his increasing isolation, he buried his feud with his old rival Muirchertach in late 1336. This diplomatic approach bore fruit as Muirchertach seemingly recognised him as his designated successor. But Brian Bán's new-found diplomacy did not end there. In 1337 he came to terms with the de Burghs in return for their recognition of his conquests. For the next few years he remained quiet, and on Muirchertach's death (1343) Brian pressed his claim to the Thomond kingship. However, Muirchertach's brother Diarmait ignored him and installed himself as king. Enraged by this slight, Brian Bán with MacNamara mounted a counter-coup and deposed the usurper within weeks. But his success was fleeting, as MacNamara switched his allegiance to Diarmait, forcing Brian's deposition. This led to further warfare, which preoccupied Brian till his death at the hands of the sons of Lorcan son of Lorcan MacKeogh in 1350 , thus ending a turbulent but remarkable career. He was succeeded by his son, the famous Murchadh na Ráithníghe O'Brien (qv).