other committees of the house, and on 27 Feb. 1641 was one of those appointed to draw up the charges of high treason against Sir Richard Bolton [q. v.], Sir Gerard Lowther, Sir George Radcliffe [q. v.], and others (Commons' Journals, Ireland, i. 217–419 passim). As early as Christmas 1640 O'Reilly was taken into confidence by Rory O'More [q. v.], with whom he had frequent conferences about the scheme for a rebellion of the catholics against the government (Memoirs of Ireland, 1767, pp. 169–90). By the end of May the plot was generally known to the Roman catholic members of the House of Commons. O'Reilly remained in Dublin till the end of the session, but in September he further discussed the matter with Maguire and Sir Phelim O'Neill [q. v.] in Cavan. He was not present at the meeting in Dublin on 5 Oct., when the scheme for the seizure of Dublin Castle was arranged, but he was assigned a part in it. On 23 Oct. Philip's nephew, Mulmore MacEdmund O'Reilly, the sheriff of Cavan, probably in concert with his uncle, raised the posse comitatus, gathered in what arms he could, and seized Farnham Castle, near Cavan, and Cavan. The next day his uncle joined him, and together they gained possession of Belturbet and neighbouring places (Henry Jones, Remonstrance of the Beginnings and Proceedings of the Rebellion in co. Cavan, 1642). O'Reilly was honourably distinguished by his conduct on these occasions; he strongly disapproved of the murders that were committed. Protestants who put themselves under his protection were safely conveyed into English quarters, and those that had been stripped were fed and clothed (Carte, Ormonde, i. 350, &c.; Gardiner, Hist. of England, x. 66), but this did not prevent various charges being brought against him in the rather questionable depositions subsequently taken (cf. Hickson, Ireland in the Seventeenth Century, passim). On 6 Nov. he headed the signatures to the remonstrance presented to the lords justices at Dublin, detailing the grievances of the rebels in Cavan. On 27 Nov. he joined the rebels with four hundred troops, and, crossing the Boyne, was present at the interview with Gormanston and other gentry of the Pale, who were induced to join the rebels by the latter's successes and their presence within the Pale.
Early in 1642 O'Reilly besieged Drogheda, but was driven away; he was more successful before the castles of Killelagh and Crohan, which surrendered to him on 4 June. On the formation of Owen Roe O'Neill's army, O'Reilly received the rank of colonel, and he was actively employed throughout the war. In 1644 he became a member of the general assembly of the confederation, and was one of its commissioners in 1646 to carry out the articles between Charles I and the confederation. He took a prominent part in the battle of Benburb on 5 June 1646. On 8 Aug. 1647 he was taken prisoner, but next year was again in active service. On 17 June he signed the declaration against the cessation. He remained a firm adherent of Owen Roe O'Neill [q. v.], whose sister he had married, and who died in his house on 6 Nov. 1649. On 9 Sept. 1649 Charles II wrote to O'Reilly urging him to do all he could to secure peace between the Irish rebels and the royalist party. In the following January he had interviews with Daniel O'Neill [q. v.] with the same object, while he was serving under Major-general Hugh O'Neill (fl. 1650) [q. v.] in the defence of Clonmel. In 1651 he was sent to relieve Fyena (i.e. probably Feeny), but, being surrounded by the enemy, narrowly escaped on horseback. In September his own house at Bellanacargy was besieged by Colonel Venables, but was relieved. In 1652 O'Reilly made his last stand in command of the garrison at the castle of Loch Uachtair. It was not until 10 April 1653 that he entered into negotiations with Colonel Theophilus Jones, and laid down his arms on condition of being allowed liberty to serve in foreign countries. He afterwards took service in the Spanish army in the Netherlands, where he had the command of a regiment. John Colgan [q. v.] dedicated to him his treatise on the works of Duns Scotus, which was printed at Antwerp in 1655. O'Reilly died at Louvain, probably about 1657.
He married Rose, sister of Owen Roe O'Neill. She is said to have been bitterly inimical to the English, and to have instigated O'Reilly to cruel measures against the captives made by the rebels. By her O'Reilly had an only son, Hugh, who married Margaret, sister of Daniel, third viscount Clare [see under O'Brien, Daniel, first Viscount Clare]. The son may be the Colonel O'Reilly who became governor of Cavan, and was killed fighting for James II in February 1690 (Luttrell, Brief Relation, ii. 17).