Ossoriense, i. 307). O'Brien was among the bishops who on 30 Aug. pronounced it ‘a deadly sin against the law of God and of his church’ to obey or proclaim the truce with Inchiquin (Confederation and War, vi. 279). He supported the excommunication and interdict fulminated by Rinuccini against those who did not agree with him, or who refused to obey him. Towards the end of the year O'Brien went to join the nuncio, who had retired to Galway, but, learning at Oranmore that he had sailed, turned aside to his own diocese. He attended the great assembly of bishops who met at Clonmacnoise in December 1649, and on 10 Feb. following wrote to some great man to say that they were united against the common enemy, though without retracting individual opinions (Spicilegium Ossoriense, i. 331). O'Brien was one of the prelates who signed the declaration of Jamestown on 12 Aug. 1650, releasing the people from their allegiance to Ormonde as lord-lieutenant, and excommunicating those who persisted in following him, and later in the same month he was one of the committee who repeated this excommunication at Galway. Ormonde left Ireland in December, leaving Clanricarde as deputy. O'Brien was one of those who at this time invited Charles, duke of Lorraine, to Ireland. The duke reported this invitation to the pope (ib. ii. 84) on 11 Feb. 1651 (N.S.), and sent some supplies to Galway, but he never came himself, and the negotiations had no real effect.
The diocese of Emly had long been overrun by the parliamentarians, and O'Brien wrote from Galway on 29 March (ib. i. 367) that the Irish cause was lost east of the Shannon, and that the enemy commanded the sea. He went to Limerick before the memorable siege, which began 2 June 1651, exhorted the people to resist, and helped to prevent them from accepting the comparatively favourable terms at first offered by Ireton. He devoted himself to the sufferers from a malignant fever which raged among the besieged, and was found in the hospital when Ireton's soldiers entered on 29 Oct. He was one of those excepted by name from pardon in the articles of capitulation, on the ground that he had opposed surrender when there was no hope of relief, and that he had been ‘an original incendiary of the rebellion, or a prime engager therein’ (Contemporary Hist. iii. 267). He was hanged on the 31st, and his head impaled over St. John's gate. By those of his own creed in Ireland, O'Brien has always been regarded as a martyr. In the acts of the Dominican chapter-general held at Rome in 1656, it is asserted, with little probability, that he refused a bribe of forty thousand aurei offered to him to quit Limerick before its investment (Hibernia Dominicana, p. 488). It is stated on the same authority, and has been often repeated, that he foretold speedy divine vengeance on the conqueror, and that Ireton, who died of fever within a month, bitterly regretted his execution, and cast the blame upon the council of war. Ireton was hardly the man to shirk responsibility, even in the delirium of fever, and neither his own despatch nor Ludlow's gives any hint of the kind.
[De Burgo's Hibernia Dominicana; Rinuccini's Embassy in Ireland, English Trans.; Cardinal Moran's Spicilegium Ossoriense; Contemporary Hist. of War in Ireland, and Hist. of Confederation and War in Ireland, ed. Gilbert; Clanricarde's Memoirs, 1744; Ludlow's Memoirs, 1751, vol. i.; O'Daly's Geraldines, translated by Meehan; Brady's Episcopal Succession; Lenihan's Hist. of Limerick. The biography of Bishop O'Brien in Myles O'Reilly's Memorials is derived from an article signed M. (? Cardinal Moran) in Duffy's Hibernian Magazine for April 1864.]