O'Rourke (Ó Ruairc), Brian Óg (1568–1604), lord of Breifne and a distinguished soldier of the Nine Years War, was a natural son of Sir Brian na Múrtha O'Rourke, lord of Breifne, and Annably, wife of John O'Credan, merchant of Sligo. The first reference to Brian comes when he was dispatched to Oxford for his education by Sir John Perrot (qv) in 1584. While there he ran up several debts as a result of his father's failure to pay for his upkeep. In 1588 his father's friend Charles Travers managed successfully to smuggle Brian from England back to Leitrim. Once back, he proved his military ability by leading troops throughout northern Connacht after his father's rebellion in April 1589. This revolt ended in defeat for the O'Rourkes, and father and son were driven into exile in Tyrconnell. Brian did not accompany his father's ill-fated flight to Scotland but lingered in the retinue of Hugh Maguire (qv), lord of Fermanagh. After his father's execution in London (November 1591) he attempted to come to terms with Elizabeth's government in April 1592, explaining his rebellion as only doing his father's bidding. Unfortunately the privy council refused to grant him his father's lands, driving him into the arms of Hugh O'Donnell (qv), lord of Tyrconnell. During the next three years, with the support of O'Donnell and Maguire, he guarded the north-western frontier of Ulster for the rebel confederacy. His centrality to the O'Neill cause is evidenced in the fact that he was the recipient of one of the letters that Capt. Alonso Cobos brought from Philip of Spain in May 1596.
O'Rourke's greatest personal desire was to obtain confirmation of his lands by letters patent from the Elizabethan government. However, this was complicated by his rebellion and by the fact that he was illegitimate – a weakness the Elizabethan officials mercilessly exploited by detaching his legitimate half-brother Tadhg from the rebels and establishing Tadhg as a ‘queen's O'Rourke’. However, government policy changed when Sir Conyers Clifford (qv) was appointed governor of Connacht (February 1597), and set out to take the strategically valuable town of Ballyshannon in Tyrconnell from O'Donnell. After his defeat by O'Donnell in July, Clifford changed his strategy and dangled the prospect of a royal grant of Breifne before Brian. In February 1598 Brian submitted at Boyle, and was granted his lands on 28 May by the queen.
The cast-off Tadhg was captured by O'Donnell, who married his sister Mary to his new pawn and used him to intimidate Brian. This worked, as Brian's fear of O'Donnell and O'Neill led him again to rebellion by the end of June 1598, campaigning with distinction in Munster and playing an important role in Clifford's defeat in the Curlew Mountains (15 August 1599). O'Rourke's fortunes continued to improve and he was reconciled with Tadhg in September 1601. When the Spanish landed at Kinsale that month, O'Rourke joined the long southward march of Irish, taking part in the battle on 24 December 1601. He returned home only to find that Tadhg, with English help, had seized his lordship.
Together with Ruaidhrí O'Donnell (qv), 1st earl of Tyrconnell, Brian tried to recover the rebel position in Connacht, but their failure to halt the English march on Sligo in June 1602 signalled the end. Afterwards he quarrelled bitterly with O'Donnell, and was the last major rebel left in the field by Christmas 1602. His position was buttressed by an influx of rebels including Domhnall O'Sullivan Beare (qv) in January 1603. However, he was defeated by a three-pronged government attack into his country, with Tadhg and Ruaidhrí O'Donnell delivering the coup de grâce. All his attempts to surrender were refused and Brian was forced to seek refuge in the forests as a simple woodkerne. For his part, Tadhg prospered and obtained a grant of Breifne and a knighthood (September 1603). Brian, tired of living on the run, hid in the Franciscan monastery of Rosserilly in Co. Galway. There he died of a fever on 28 January 1604 while awaiting a ship to Spain. Throughout Brian's life his illegitimacy proved a fatal weakness that was used to undermine his position as successor to his father and ultimately led to his undoing.