O'Neill (Ó Néill), Brian (d. 1562), 2nd baron of Dungannon , O'Neill claimant, was eldest son of Matthew (Feardorcha) O'Neill (qv), 1st baron of Dungannon, and his wife Joan, daughter of Cú Chonnacht Maguire (qv), and was the short-lived rival of his uncle, Shane (Seaán) O'Neill (qv). Nothing can be deduced about his early life from Irish records or those of the Tudor government. The first mention of Brian comes in a letter written by his grandfather Con Bacach O'Neill (qv), 1st earl of Tyrone, to Elizabeth I in June 1558, according to which James MacDonnell (qv) had imprisoned Brian in Scotland since the beginning of 1556. The earl desired Elizabeth to intervene on his behalf with the government in Dublin to effect Brian's release through ransom or rescue. Brian seemingly returned to Ulster at the close of 1558 or in the opening months of 1559.
In his absence, his father had been assassinated on the orders of Shane. As Matthew's eldest son, Brian succeeded to the barony of Dungannon. He adopted his father's loyalist stance and pursued his claims to the earldom of Tyrone by lobbying the government to eject Shane from Tír Eóghain. Concurrently, he waged war to little effect against Shane and Turlough Luineach O'Neill (qv). In August 1560 Elizabeth ordered the subjugation of Shane and the installation of Brian as overlord of Tír Eóghain. Shane's march through the southern Ulster lordships (July 1560) intimidated his Irish enemies and the alliance dissolved. In 1561 Shane smashed the encircling ring by capturing An Calbhach O'Donnell (qv) in May, and inflicted a defeat upon the government army two months later. These reverses caused the Dublin government to reopen negotiations with Shane. During the negotiations Brian petitioned Elizabeth (December 1561). Shane presented himself before Elizabeth in January 1562, and argued that Brian's claims were not legal as his father Matthew had been a Kelly, not an O'Neill. On 13 March 1562 Elizabeth ordered Brian to appear before her, but countermanded her decree some weeks later. Shane then arranged for Turlough Luineach to kill Brian, which he did on 12 April 1562.
Throughout Brian's brief career, he was too weak militarily to wrest the overlordship of Tír Eóghain from Shane. His persistent claims to the earldom of Tyrone ultimately cost him his life. Historically, Brian's career is a footnote, but it cannot but have influenced his brother and successor, Hugh O'Neill (qv).