The father, who came originally from County Clare, was descended, although himself a Protestant, from a Roman Catholic branch of the great O'Brien family, which had been deprived of its property by the penal laws; he was well educated, but more imprudent than provident.
On 9 March in the same year, he was raised by the Prime Minister, Sir Robert Peel to the bishopric of the united dioceses of Ossory, Ferns, and Leighlin.
O'Brien was a daily worshipper in St Canice's Cathedral, but he seldom preached or spoke except at the meetings of the church education society, of which he was an active champion.
In 1850 appeared his ‘Tractarianism: its present State, and the only Safeguard against it.’ To the disestablishment of the Church of Ireland O'Brien maintained a well-sustained resistance, and Archbishop Trench acknowledged much aid from his advice in the course of the struggle.
When disestablishment came, O'Brien helped to reorganise the church, and moderated the zeal of his evangelical friends in their efforts to revise the prayer book in accordance with their own predilections.
On 19 December Archbishop Trench described him, when addressing the clergy of the diocese assembled to elect a successor in the see, as a fit representative of the ideal anēr tetragōnos, i.e. the philosopher's four-square man, able to resist attack from whatever quarter made, and his successor, Dr. Robert Gregg, in his primary charge, spoke of O'Brien's ‘unvarying consistency, calm judgement, and chastened self-restraint.’ His personal appearance was dignified and imposing.