Career
He was beatified among the Seventeen Irish Martyrs by Pope John Paul II on 27 September 1992. O'Brien hailed from Cappamore, County Limerick and joined the Dominicans, studying 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. 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. He died a martyr, following his execution in Limerick.
A large backlighted portrait of him is on display in St. Michael's Church, Cappamore, Co. Limerick, which depicts him during The Siege of Limerick.