O'Brien, Barnabas (Bryan, Barnaby) (c.1590–1657), 6th earl of Thomond and MP, was the younger son of Donough O'Brien (qv), 4th earl of Thomond, and his second wife, Lady Elizabeth Fitzgerald, daughter of Gerald Fitzgerald (qv), 11th earl of Kildare. He attended Brasenose College, Oxford, 1605–7, matriculating with his older brother Henry (see below), baron of Ibrackan, in February 1605. The college principal, Dr Singleton, informed their father in August 1607 that he should not keep his sons there, as their desires were set on home and other things. Barnabas entered Lincoln's Inn in 1613, then a typical educational route for younger sons of the Irish nobility and gentry. He married Mary Crighton (1592–1675), youngest daughter of Sir George Fermor and widow of Robert Crighton, on 17 July 1615. They had one son, Henry (1621–91), who succeeded Barnabas as 7th earl of Thomond, and one daughter, Penelope, who married Henry Mordaunt, 2nd earl of Peterborough.
Although he had provided for Barnabas's education, Donough O'Brien, who died in 1624, left Barnabas nothing in his will. He appointed Henry his sole executor and bequeathed the entire earldom to him, expressly revoking any prior arrangements whereby lands in Co. Clare had been entrusted to Barnabas's use. Barnabas served as MP for Coleraine in the 1613–15 parliament, and in 1634 was nominated MP for the boroughs of Carlow and Ennis, opting to sit for Ennis. In 1640 he took his seat in the Irish house of lords, having succeeded his brother Henry to become 6th earl of Thomond in 1639. He was appointed governor of Clare by privy seal on 21 November 1639, and became a member of the Irish privy council.
Thomond's time as earl was marred by financial and political troubles. The earldom he inherited from his brother, Henry, was seriously impoverished by comparison with its prosperity under the careful management of his father. When plantation became a live issue in the mid-1630s, Lord Deputy Thomas Wentworth (qv) pondered whether favour should be shown to the new earl, ‘who merited nothing, as to his brother who did’ (O'Donoghue, 258). The earldom suffered considerable disruption in the aftermath of the rising of 1641. Many of the protestant settlers who had been introduced to the Thomond estate during the lifetime of the 4th earl were forced to leave, the 6th earl proving powerless to protect them. Thomond remained loyal to the earl of Ormond (qv) during the ensuing war, thereby alienating himself from many of his kinsmen and tenants. In 1646 he left Bunratty permanently and moved to live in Great Billing in Northamptonshire. He eventually recovered some of his Irish losses, but remained impoverished. In his will, dated 1 July 1657, he made explicit reference to his lack of wealth as a result of the heavy charges imposed on the estate by his brother, Henry. He expressed a wish to be buried with his father and brother in Limerick, or, if he died in England, in Great Billing church. He died in November 1657 and was buried at Great Billing on 15 November. He was survived by his wife, Mary, and in his will, proved in London on 6 February and in Dublin on 28 April 1658, he expressly enjoined on his son and heir the responsibility to tend and cherish his mother. A portrait of the 6th earl, painted by C. Johnson in 1643, survives at Petworth House.
Henry O'Brien (d. 1639), 5th earl of Thomond , nobleman, was educated at Eton College and TCD, as well as Brasenose College, Oxford. He was appointed governor of Clare on 19 March 1616, when his father, Donough, resigned the post in his favour. This office was a device to exempt the earldom of Thomond from the jurisdiction of the provincial presidency. He became commissioner for Munster in the absence of his father in 1618 and again from 19 September 1624 to May 1625 following the death of his father. In contrast to his brother, Barnabas, he probably enjoyed some of the best of times at Bunratty castle, where he resided as earl of Thomond from 1624 to his death in 1639: these were years of relative peace and prosperity. The Dublin administration approved of Henry's ‘honesty and ability and soundness in religion’ (CSPI, 1611–14, 119). His loyalty was rewarded in the 1630s when Sir Francis Aungier (qv) advised Lord Deputy Wentworth to proceed with a plantation of Thomond, while ensuring ‘that nothing should be done therein to the prejudice of the noble earl of Thomond, nor his brother Sir Barnaby Brien, being both good and firm protestants, and all their children’ (letter book 1, June 1632, Strafford papers, Sheffield Archives). The 5th earl enjoyed Wentworth's favour, and in return left him a gift to the value of £100 in his will. Thomond was appointed to the Irish privy council in September 1633, though his public career was largely ineffective.
Henry married Mary, daughter of Sir William Brereton, on 13 July 1608. They had five daughters, Mary, Elizabeth, Margaret, Ann, and Honora, for whom the 5th earl strove to arrange suitable marriages, financed by mortgaging extensive portions of the Thomond estate for immediate cash gain. Having no male heir, Henry provided generously for his daughters in his will, drawn up three weeks before his death. Thus, when his brother, Barnabas, succeeded to the earldom in 1639 he was left with a heavily indebted estate, and the outbreak of rebellion in 1641 meant that it could not meet all of the financial commitments that had been entered into. Henry O'Brien, 5th earl of Thomond, was buried in late April 1639 in the elaborate tomb he had erected for his father in Limerick cathedral, after a funeral that he had stipulated should be ‘without any vain ostentation’.