Fitzwilliam, Oliver (d. 1667), 2nd Viscount Merrion and earl of Tyrconnell , landowner and army officer, was the eldest son of Thomas Fitzwilliam, 1st Viscount Merrion, and his wife, Margaret, daughter of Oliver Plunkett, fourth Baron Louth. The Fitzwilliams were an important catholic Old English family, with extensive land holdings in Dublin and other counties of the Pale. Their long and loyal service to the crown was rewarded when Sir Thomas Fitzwilliam was raised to the peerage by Charles I in 1629, as Baron Fitzwilliam of Thorncastle and 1st Viscount Merrion. Oliver and his brother Richard were admitted to Gray's Inn as students in August 1628 and, some time before 1641, Oliver was commissioned as a colonel in the French army to raise men in Ireland for service in France. Fitzwilliam received his commission largely through the influence of Ormond (qv), to whom he was related. Prior to the outbreak of rebellion in 1641, he recruited 3,000 Irishmen to serve in France, and he raised a further 400 men in 1642.
Early in 1645 Fitzwilliam entered negotiations with Queen Henrietta Maria in France, seeking concessions for Irish catholics. In return for his undertaking to raise an army of 10,000 men to fight for Charles I, the queen promised her assistance in the removal of penal laws, the free practice of religion and a free parliament in Ireland. All parties to the agreement were sceptical as to its feasibility. They doubted the king's ability to agree to such terms, and the queen was doubtful that Fitzwilliam could raise so many men in Ireland. Nevertheless, he went to England, in time to fight on the royalist side at Naseby on 14 June 1645 and, on 29 June, he arrived in Ireland with a personal letter of recommendation from Charles I to Ormond, suggesting that his services should be accepted. When the scheme failed, he became involved in promoting the Glamorgan (qv) peace in late 1645.
In 1646 Fitzwilliam took part in the confederate expedition to Connacht, under his uncle General Thomas Preston (qv). While in Connacht, he commanded 500 infantry from the regiment of Sir James Dillon (qv), as well as 200 cavalry. His troops were involved in the seizing of livestock in Co. Roscommon, where he successfully defeated a protestant force sent to impede him. When Preston advanced on Dublin later in 1646, Fitzwilliam kept Ormond informed of conditions in the Leinster army, urging him to agree religious terms with the confederates. By February 1647, he was in Paris and reporting to the confederate supreme council, advising them of his loyalty and of the urgent need to take Dublin quickly. Fitzwilliam's profession of loyalty was in response to rumours that he had been sent to France to avoid arrest, as his service to Ormond had put his loyalty in doubt.
In November 1649 Fitzwilliam went to England in an effort to protect his estates. He was arrested, however, and, though ordered to leave the country within eight days, he did not depart for France until 1650. His wife received a pass to return to England in the summer of 1651, where she successfully petitioned the council of state on behalf of her husband. The following October, Fitzwilliam was granted permission to return to England on provision of two sureties and, once there, he began protracted negotiations to preserve his Irish estates. The process was greatly aided by two factors: first, by the fact that many of his lands had been mortgaged before the outbreak of the 1641 rebellion and, second, by his marriage to Eleanor Holles. His marriage provided him with influential connections through his brothers-in-law, Denzil Holles and John Holles, 2nd earl of Clare, both prominent parliamentarians. Eleanor's marriage portion of £6,000 was protected in trust by Denzil and, because Fitzwilliam was a delinquent, the trust could use the money to repay mortgages on his estates and prevent confiscation. In 1654 Oliver Cromwell (qv) exempted the lands held by Eleanor's trustees from sequestration, though there were complaints, in 1655, that she had not received the full benefit of the ordinance. In this manner, Fitzwilliam managed to protect most of his lands during the commonwealth period. Indeed, the extent of his influence can be seen in the somewhat comical incident of 1655, when Fitzwilliam offered to use his best efforts with Cromwell, on behalf of the arrested parliamentary general Edmund Ludlow (qv).
After the restoration, and in recognition of his loyalty to the king, Fitzwilliam was granted an order restoring him to his lands in December 1660. On 20 April 1661, he was created earl of Tyrconnell and he took his seat in the house of lords, by proxy, on 9 July 1662. In March 1663, however, he was declared ‘nocent’ under the Act of Settlement, largely as a result of a letter he had written, in 1647, to the confederate supreme council encouraging the prosecution of the war in Ireland. The judgment may also have been due, in part, to his unpopular personality and his success in preserving his lands during the 1650s. He was pardoned by the Irish council on 30 November 1663, though Ormond supported the pardon only because of his connections with the Holles family. Over the next few years, Fitzwilliam reconstructed and restored the family seat at Merrion castle, which had been destroyed during the war. He died 11 April 1667 at Merrion and was buried at Donnybrook church the following day.
Fitzwilliam married three times; his first wife was Dorothy Brerton, and Eleanor Holles, whom he married between 1642 and 1647, was his third wife. As he had no children, the earldom of Tyrconnell became extinct on his death, and his estates and other family honours passed to his younger brother, William (d. 1675).