O'More, Brian Riabhach (c.1570–1598), noble of Leix (Laoighis) and military commander, was a natural son of Rory (Ruaidhrí Óg) O'More (qv) (sl. 1578), lord of Leix, and was the only issue of a relationship with a woman of the O'Carrolls of Ely. In November 1572 Rory married his second wife, Margaret (Mairghréag) O'Byrne of Glenmalure (sl. 1577), whose family were especially keen that one of her children should eventually succeed her husband as lord of Leix. This may explain why Rory never acknowledged Brian as his son. Thus Brian grew up believing that he was the son of the leader of the Mac Lochlainn O'Mores, probably Ross O'More. After the killing of Rory by the MacGillapatricks of Upper Ossory in July 1578, Ross briefly became lord of Leix. His reign over the O'Mores was short, as James Meagh O'More (fl. 1583) was universally recognised as lord of Leix by 1580. Nothing substantive is known of Brian Riabhach's activities except that he was living on the borders of Ely and Leix during the 1580s. Around this time he earned the enmity of Sir Charles O'Carroll of Ely (fl. 1595). According to Sir Charles, Brian Riabhach's life took a dramatic twist in 1589, when his mother told him that Rory was his real father. This had a major affect upon the young man, transforming him from a vassal into one of the most prominent claimants to the O'More leadership. No doubt Brian Riabhach's emergence as a major force in Leix politics irked his half-brother Uaithne. Since Rory's death Fiach O'Byrne (qv) (sl. 1597) had been grooming Uaithne to take his father's place. And Brian Riabhach may have felt aggrieved by the favouring of the younger Uaithne's claims over his, considering that both of them, as recognised sons of Rory, had an equal claim to the lordship. In 1592 he went to London, probably en route to join Spanish forces in Flanders. In Cheapside in London he encountered his enemy; Sir Charles O'Carroll immediately went to the authorities and had him arrested and thrown in jail. It is unknown how long Brian Riabhach remained imprisoned, but he may have secured his release by promising service against O'Byrne and Uaithne. In July 1594 a John Byrd conveyed Brian's offers to Sir Robert Cecil, saying that O'More would deliver the head of Fiach and O'Byrne's secret correspondence with Hugh O'Neill (qv), 2nd earl of Tyrone. However, Brian Riabhach did not keep his promise to attack Fiach and Uaithne; on the other hand, he did not actively support or oppose the activities of Fiach and his protégé. His relations with Fiach do not seem to have been strained; Fiach wrote to him in November 1596, urging him to fight for his catholic faith.
Fiach's killing on 7 May 1597 was a huge setback to the Leinster Irish. However, it propelled Brian Riabhach to the forefront of the leadership of the Leinster catholics in arms. The untenable military situation in Leinster perhaps spurred Tyrone to encourage Domhnall MacMurrough Kavanagh (qv) and the sons of Fiach to withdraw into the safety of Ulster. They were joined on their trip northward by Uaithne and Brian Riabhach. It seems that during his sojourn in Ulster the English moved against his supporters. On the authority of the Dublin government, Sir Charles O'Carroll arrested Brian Riabhach's uncle, Ken Buidhe O'Carroll. It was only in October 1597 that Tyrone considered it safe enough in Leinster for the Leinster émigrés to return. On 18 October it was reported that 800 men under Brian Riabhach were raiding within six miles of Dublin before attacking the English of Wexford. While Brian Riabhach continued to attack the English in east Leinster before raiding into Upper Ossory, Uaithne was sent to Leix to attack the plantations, winning a major engagement on 7 December against the troops of Warham St Leger (qv) (d. 1600), the governor of Leix. Despite Tyrone's truce of December 1597 with the English, fighting continued in the midlands, although it seems to have largely ceased in east Leinster. Another side of Brian Riabhach can be seen in his treatment of a captured O'Farrell spy in the pay of the English. Usually the spy would have been hanged, but because his master had regularly visited Brian Riabhach during his imprisonment in London, the O'More leader repaid his kindness by treating the man humanely.
During Easter 1598 Brian Riabhach pushed into north Tipperary, spending the holiday in the O'Meagher lordship of Ikerrin. There he was attacked by James fitz Edward Butler (d. 1598) without the consent of Thomas Butler (qv), 10th earl of Ormond. But the catholic forces overcame James fitz Edward, capturing him. Within a week, though, Brian Riabhach handed his captive into the custody of Ormond. However, Brian's end was not long in coming. On 6 July he was mortally wounded while resisting Ormond's incursions into Leix, dying four days later of his wounds. This left Uaithne as the principal catholic commander in the midlands and in Leinster. Brian Riabhach is not recorded as having married and apparently died childless.