O'Brien (Ó Briain), Conchobhar (d. 1539), lord of Thomond, was eldest son of Toirdhealbhach Donn O'Brien (qv) (d. 1528), lord of Thomond, and Raghnailt, daughter of Seaán MacNamara. Nothing is known of his early life, but he married first (a. 1515) Annabella, daughter of MacWilliam Burke; they had one son, Donnchadh. During the 1520s the O'Briens allied with Gerald FitzGerald (qv), 9th earl of Kildare, during his countrywide dispute with Piers Ruadh Butler (qv), the disputed earl of Ormond. Conchobhar's brother Tadhg was shot dead by the soldiers of Piers Ruadh during this struggle in 1523.
On the death of his father (1528) Conchobhar became lord of Thomond while his brother Donnchadh (d. 1531) became tánaiste. He quickly came to the notice of the Tudor government when he allied with Cormac Óg MacCarthaigh (Mac Carthy) (qv) (qv) sometime in 1528. Conchobhar's reign was to prove turbulent and testing as the Kildare–Butler struggle reached its climax during the early years of the 1530s. By this date he had married Ellen, daughter of James FitzGerald, 13th earl of Desmond (qv). This union, which produced a further five sons and at least one daughter, cemented his links with the FitzGeralds of Desmond.
When Thomas FitzGerald (qv), Lord Offaly, renounced his loyalty to Henry VIII in June 1534, Conchobhar followed suit and in a letter offered his allegiance to the emperor Charles V in July 1534. The subsequent war between Offaly and the government bitterly divided the O'Brien dynasty. Conchobhar's eldest and estranged son, Donnchadh, turned against his father and supported his father-in-law, Piers Ruadh. After the fall of Maynooth (March 1535) Offaly fled to Conchobhar, who gave him sanctuary till a ship could be arranged to take him to Spain. While Offaly hid in Thomond, a plot was devised by Edmund Sexton, mayor of Limerick, with Donnchadh to capture the FitzGerald leader. Nothing came of it, and Offaly decided to return to the midlands, leaving much of his silver plate in the custody of Conchobhar. Despite the surrender (August 1535) of Offaly, now 10th earl of Kildare, Conchobhar continued in the field and thwarted the attempts of Tudor armies to penetrate Thomond. In October 1535, however, his attitude softened and he wrote to Henry VIII, excusing himself for his involvement in the rebellion.
His words of contrition did not reflect his actions, and campaigns were led against him and his ally Sir James fitz John Fitzgerald (qv) (d. 1558) of Desmond by Lord Leonard Grey (qv), lord deputy of Ireland, in July 1536. Grey's army, guided by Donnchadh, took (after a brief siege) the fortress of O'Brien's Bridge. Luck intervened to gain Conchobhar a respite when the army refused to enter Thomond because of non-payment of wages. As a result Thomond was to remain a safe haven for the Geraldines for a further two years, sheltering young Gerald FitzGerald (qv), later 11th earl of Kildare, and Brian O'Connor Faly (qv) in 1537.
In July 1538 Conchobhar strengthened his regional position by marrying his daughter to O'Carroll of Ely, but by the close of the month both he and Sir James fitz John FitzGerald, now 14th earl of Desmond, realised the futility of the FitzGerald cause and submitted to Grey. With the lord deputy and Desmond, he then campaigned against his brother and dynastic rival Murchadh O'Brien (qv), who still adhered to the FitzGerald cause in August 1538. In 1539 Conchobhar failed to prevent the succession of Murchadh to the lordship of Thomond. However, the success of his reign is commented on by the annalist who describes Thomond as ‘wealthy and prosperous’ in Conchobhar's obit of 1539.
Conchobhar's deft political skill and military ability helped to maintain the power of Thomond throughout the turbulence of the Kildare rebellion and its aftermath. However, his realisation that the FitzGerald cause was lost by 1538 signalled a change in his policy and the development of better relations with the government. This change in political tack ultimately resulted in the preservation of many of the O'Brien lands.