O'Callaghan, Thomas Alphonsus (1839–1916), catholic bishop, was born 9 May 1839 in Sullivan's Quay, Cork, son of Michael O'Callaghan and Margaret Palmer, both from Cork. He was educated at North Monastery CBS, O'Connor's school, Sunday's Well, and later at O'Sullivan's classical school, South Mall and at St Mary's grammar school, Pope's Quay. He joined the Dominicans at St Mary's, Tallaght, Co. Dublin in 1857 with the name Alphonsus Mary and was professed on 17 November 1858. He began his philosophical studies in November 1859 in Rome at the College of St Thomas at the Minerva, residing at San Clemente while pursuing this course. He was a student in the Dominican convent of La Quercia, Viterbo in 1861 where he secured his licentiate in theology, and was ordained there in 1864 by Cardinal Bedini, bishop of Viterbo.
Returning to Ireland he taught philosophy for six years at Tallaght, at the same time acting as sub-master of novices. He was in Cork in 1870 when he contracted smallpox, recovering in 1872, and became prior of the Dominican house in Galway and of St Catherine's, Newry in 1874. He was summoned to Rome and became master of novices and lecturer at San Clemente from 1870 to 1880. He had strong personal and confessional links in Roman circles through his friendship with Cardinal Allesandro Barnabo, cardinal prefect of Propaganda Fide and was for a time personal confessor of Pius IX. From February to August 1880 he was again prior in Newry. He was consecrated titular bishop of Zambezi by Cardinal Simioni in the basilica of San Clemente on 29 June 1884 and made prior of San Clemente in August, a position he held until his appointment as coadjutor to Bishop William Delany (qv) of Cork in 1884. Bishop Delany did not welcome his coadjutor and he was obliged to live with his sisters, Margaret and Hannah, at 34 South Mall, Cork depending on them for maintenance, until he succeeded Delany on 14 November 1888.
O'Callaghan issued a pastoral letter in February 1901 warning against the dangers of socialism, based, as he saw it, upon ‘false philosophy’ and ‘false principles of morality’. During his episcopate he built new churches and improved old ones, expanding the Christian and Presentation Brothers' schools in his diocese. He also founded St Angela's High School for girls on St Patrick's Hill. His attitude to QCC became gradually more favourable and in 1905 he gave official leave for the formation of a students' association, appointing two chaplains. On the establishment of the NUI and its constituent colleges in 1908, he became a member of the UCC governing body.
Many Cork charitable institutions owed their existence to his zeal. He introduced the Little Sisters of the Assumption to Blackpool, Cork, and was responsible for the design and completion of the home for convalescents at the Bon Secours, College Road, and for the novitiate attached to the convent. He created a central novitiate for the Sisters of Mercy in the diocese in 1914 and supported building a convent at College Road by the Dwyer family, for the contemplative Order of Collatines (Poor Clares). He laid the foundation stone for the Honan chapel at UCC on 18 May 1915. From 1908 he was sympathetically involved in the case of Nellie Organ (qv), a pious child whose cause for beatification he promoted and which may have been stalled by his death. He bequeathed all the household effects in the bishop's palace, where he had died on 14 June 1916, to his sister Margaret.