O'Neill, Sir Phelim (c.1604–1653), soldier and politician, was eldest son of Turlough O'Neill and Catherine, daughter of Turlough MacHenry of the Fews. Phelim inherited lands in Tyrone and Armagh from his grandfather Sir Henry O'Neill, who was killed in action during the rebellion of Sir Cahir O'Doherty (qv) in 1608. The lord deputy, Sir Arthur Chichester (qv), divided the inheritance during Phelim's minority, and it was not until 1629 that he received a new patent vesting all the lands in him. O'Neill entered Lincoln's Inn with his younger brother Turlough in June 1621, and spent a number of years in London. On his return to Ireland, he ingratiated himself with the Dublin government, serving as a commissioner for army funds and justice of the peace, as well as replacing his native tenants with protestant settlers from England. In 1639 his kinsman Randal MacDonnell (qv), 2nd earl of Antrim, persuaded the lord deputy, Thomas Wentworth (qv), to knight O'Neill. During the summer of 1641 he replaced Thomas Madden as MP for the borough of Dungannon, and served as an active member of the parliamentary committee established to discuss the transport of troops abroad. Described in one account as ‘a well bred gentleman . . . as free and generous as could be desired’ (DNB), O'Neill lived in an extravagant style, well beyond his means. A serious debt problem (his lands were mortgaged for over £13,000) almost certainly disposed O'Neill to contemplate radical action. He established contact with his kinsman Owen Roe O'Neill (qv) in Flanders, and in September 1641, after a meeting with Philip MacHugh O'Reilly (qv) and Conor, Lord Maguire (qv), Phelim committed himself to the idea of an armed rebellion. The plotters did not envisage an overthrow of the existing order but sought instead liberty of religion and the recovery of traditional estates by the Ulster Irish.
On the night of 22 October 1641 O'Neill seized the strategic fortress of Charlemont, and within a few days his forces controlled much of south Ulster. On 4 November he published a commission from Charles I (subsequently revealed as a forgery) authorising him to take arms in the defence of the king. His professed loyalty to Charles subsequently encouraged the catholics of the Pale to join forces with the Ulster rebels. After the defeat of government troops at Julianstown, Co. Meath (29 November 1641), by Philip MacHugh O'Reilly, the Pale leadership invited O'Neill to assume command of the rebel forces blockading Drogheda. He gladly accepted, having suffered a setback the previous day at Lisburn, thwarting any attempts to move against Carrickfergus and Belfast. The blockade of Drogheda lasted until March 1642, when a relief force from Dublin, led by James Butler (qv), 12th earl of Ormond, forced O'Neill to return north. The arrival of Owen Roe O'Neill (July 1642), effectively replacing Phelim as ‘chief of his name’, gave a fresh impetus to the rebellion in Ulster. The following month a provincial assembly appointed Owen Roe lord general of Ulster, and Phelim colonel of the cavalry, with the symbolic title of ‘lord president’ of Ulster. Intense rivalry between the two men erupted periodically, forcing leading clerics, including the papal nuncio, GianBattista Rinuccini (qv), to act as intermediaries on a number of occasions.
Phelim's first wife, a daughter of Lord Iveagh, had died in September 1641, and he next married Louise, the Dutch-born daughter of Owen Roe's great rival, Thomas Preston (qv), general of the confederate Leinster army. This match strengthened Phelim's ties with the confederate peace faction, who consistently opposed Owen Roe and his clerical supporters. O'Neill attended the first general assembly in Kilkenny in October 1642 and assisted Patrick Darcy (qv) in drawing up the confederate ‘model of government’. He concentrated primarily on military matters for the next few years, and fought with distinction at the battle of Benburb (June 1646). O'Neill supported the peace deal with Ormond, declared in August, but surprisingly agreed to serve with Owen Roe in the supreme council established by Rinuccini after the clergy successfully opposed the treaty. However, tensions in the Ulster camp – between those men who had held land in 1641 and the returned exiles – increased as the confederate association spiralled into civil war. The penultimate general assembly (November 1647) appointed O'Neill to the supreme council, and he supported the truce agreed in May 1648 with the royalist Murrough O'Brien (qv), Lord Inchiquin. O'Neill and a number of key Ulster officers, including Miles O'Reilly and Alexander MacDonnell (qv), now refused to serve under Owen Roe. Throughout the ensuing summer O'Neill joined forces with Preston and Ulick Burke (qv), marquis of Clanricarde, against Owen Roe and the papal nuncio. He attended the final general assembly (September 1648), and accepted a nomination to the committee of treaty to negotiate a fresh settlement with Ormond.
Despite his dispute with Owen Roe, Phelim did not entirely ignore the interests of the dispossessed of Ulster. He joined with the moderate faction in Kilkenny in seeking better terms, particularly on the issues of plantation and religion, than those offered by Ormond in 1646. The treaty talks reached a successful conclusion on 17 January 1649, although Owen Roe remained outside the new royalist/confederate coalition. In November 1649 Owen Roe died and Phelim failed in his attempt to be elected general of the Ulster forces. He served instead under the new commander, Heber MacMahon (qv), bishop of Clogher. After the disastrous defeat by the forces of the English parliament at Scarrifhollis, Co. Donegal (June 1650), O'Neill fled to Charlemont but the fortress surrendered on 6 August. He was given permission to leave Ireland, but resumed guerrilla warfare against parliamentary forces in Ulster. In February 1653 Philip Roe MacHugh O'Neill betrayed his hiding-place in Tyrone, and he was brought to trial in Dublin before the high court of justice. The court offered to pardon him if he implicated Charles I in the Ulster rebellion of 1641, but O'Neill refused to comply. He was executed on 10 March 1653 for his alleged role in the massacres of protestants in 1641; the authorities impaled his remains on the gates of Dundalk, Drogheda, and Dublin. He was survived by his third wife (m. 1649), Lady Jean Gordon, daughter of Lord Huntley; their son Gordon sat in the Jacobite parliament in 1689 (representing Co. Tyrone), and played an active role in the Williamite wars.