O'Sullivan, Mortimer (1791–1859), Church of Ireland clergyman and polemicist, was born in Clonmel, Co. Tipperary, second son of John O'Sullivan, schoolmaster. Although they were catholics, Mortimer and his elder brother Samuel (qv) attended the school of a protestant clergyman, the Rev. Dr Richard Carey, under whose influence the brothers and their friend William Phelan converted to the Church of Ireland. In June 1811 O'Sullivan entered TCD, where he was a prominent member of the College Historical Society, and was awarded a scholarship (1813); he graduated BA in 1816, and was ordained in the same year. He returned to Tipperary to work as a schoolmaster and curate in Tipperary town, before taking up a teaching appointment at the Royal School in Dungannon. In the early 1820s he was a curate in St Stephen's chapel, Dublin, and married (1824) Elizabeth Bloomfield Baker, a popular evangelical preacher. In December 1827 he was appointed to St Audoen's, Cornmarket, resigning in August 1830 when he succeeded Phelan as rector of Killyman, Co. Tyrone. He graduated MA (1832) and DD (1837). In 1849 O'Sullivan became a prebendary of Armagh, and in 1853 was appointed rector of Tandragee, Co. Armagh. During his last years he also served as chaplain to the earl of Carlisle (qv), lord lieutenant of Ireland.
He maintained a lifelong interest in the catholic church in Ireland, and became actively involved in the religious controversies of the period. Unusually for a religious convert, he maintained contact with catholic members of his family, and was initially regarded as politically liberal. In his book Captain Rock detected (1824), a reply to Captain Rock, by Thomas Moore (qv), he criticised Irish landlords as the cause of the disaffection widespread in the south of Ireland, countering Moore's attack on anglican clergy and the tithe levy. O'Sullivan proposed that the government invest in the education of catholics and improve their living conditions. In 1825 he gave evidence before the select committee on the state of Ireland, and (with Phelan) published his testimony in A digest of evidence on the state of Ireland in 1824–5 (2 vols, 1826). O'Sullivan has been suggested as the original model for ‘Murty O'Mulligan’ in Moore's satirical 1818 collection of verse letters entitled The Fudge family in Paris (O'Donoghue). He also reportedly inspired Moore's Travels of an Irish gentleman in search of a religion (1833), to which O'Sullivan promptly responded with A guide to an Irish gentleman in his search for a religion (1833).
In the 1830s O'Sullivan's political and religious outlook hardened as he became increasingly anxious at the spread of ultramontanism among Irish catholic clergy. In 1835 he gave evidence before the select committee on Orange lodges, stating that their activities preserved peace in Ulster. Later that year he became embroiled in a controversy with Archbishop Daniel Murray (qv), who accused him of misquoting him to the committee. O'Sullivan recounted this episode in his Case of the protestants in Ireland stated (1836). He toured Scotland in the later 1830s, addressing protestant associations; reports of these meetings were collected in Romanism as it rules in Ireland (1840), published in conjunction with his associate Robert James McGhee. O'Sullivan later published Theory of developments in Christian doctrine (1846), a reply to Apologia pro vita sua, by John Henry Newman (qv), and in 1851 was appointed Donnellan lecturer at TCD. In 1853 he and the Rev. John Charles Martin (1797?–1878) edited a collection of his brother's unpublished articles, entitled Remains of the Rev. Samuel O'Sullivan (3 vols, Dublin, 1853). O'Sullivan was a renowned preacher, whose eloquence was praised by William Carleton (qv), among others, and several of his speeches and sermons were published. He died 30 April 1859 at his Dublin residence in Gloucester St., and was buried in the family plot in Chapelizod churchyard. Some of his correspondence is held in the National Library of Scotland, Edinburgh.