Born in Limerick, O'Brien joined the Dominicans and studied at Toledo and Limerick. He was consecrated bishop of Emly by Giovanni Battista Rinuccini. During the Irish Confederate Wars, like most Irish Catholics, he sided with Confederate Ireland. The bishop would treat the wounded and support Confederate soldiers throughout the conflict. O'Brien was against a peace treaty that did not guarantee Catholic interests in Ireland and signed the declaration against the Confederate's truce with Inchiquin in 1648 who had committed atrocities against Catholic clergy and civilians, and the declaration against the Protestant royalist leader Ormond in 1650 who, due to his failure to resist the Cromwellian conquest of Ireland was not deemed fit to command Catholic troops.
In 1651 Limerick was besieged and O'Brien urged a resistance that infuriated the Ormondists and Parliamentarians. Following surrender he was denied quarter and protection. Major General Purcell, Father Wolf and O'Brien were brought before a court martial and ordered for execution by General Henry Ireton. On 27 September 1992, O'Brien and sixteen other Irish Catholic Martyrs, including Dermot O'Hurley, were beatified by Pope John Paul II.
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