O'Callaghan, John Joseph (c.1838–1905), architect, was born in Co. Cork. As a youth he received his first architectural training from John Benson (qv), the county surveyor, before becoming a clerk in the office of Thomas Deane (qv) and Benjamin Woodward (qv), architects in Dublin. O'Callaghan may have been related to Daniel O'Callaghan, who was also a clerk at Deane & Woodward at about the same time. In 1856 John was sent to Oxford to oversee the construction of the Union building. He may also have played a part in the building of the Oxford University natural history museum (1854–60). At Oxford he relished the opportunity to examine medieval architecture, and sketched the colleges in meticulous detail. For the rest of his career he studiously adhered to the precepts of the Gothic style; in his view this was the only ‘true style’ in architecture. He was employed by Deane & Woodward between 1856 and 1871, when they were at the height of their powers, and he may have contributed in some measure to a number of important buildings in Dublin such as the Kildare Street Club (1858–61) and the Museum Building, TCD (1854–57). In 1871 he set up his own practice at 16 Nassau St., Dublin, and entered a number of competitions in England and Ireland to build important public buildings and churches; he was on the short list for Winchester town hall, Hampshire (1871), and appears to have clinched the contract for the Chorley town hall, Lancashire (1872). Other contracts soon followed to build many Roman catholic churches all over Ireland, such as Clifden (1872–4), Clara (1876–83), Castlebar (1876–7), Mountmellick (1878), and Castletown Geoghegan (1883), as well as church schools and convent chapels such as the O'Brien Institution, Clontarf (1880–83). Like Pugin and Burges – the great English gothicists – O'Callaghan placed great emphasis on small details such as polychrome brickwork, finials, turrets, ornate arches, carved roundels, and the juxtaposition of different materials such as brick and granite to enliven his designs. His understanding of medieval detail meant that he won a number of contracts to remodel existing buildings and design altarpieces and fountains.
O'Callaghan also applied his Gothic style to a range of commercial buildings including banks, public houses, hotels, theatres, and shops. His work on the Olympia Theatre, Dublin (1879–81) was largely remodelled a decade later. Outstanding surviving examples in Dublin include the highly ornate red-brick Dolphin Hotel façade (1898) and the stone-faced London and Lancashire Fire Insurance Company (1894) building which stands on a prominent position overlooking O'Connell bridge like a miniature French chateau. By the 1890s his Gothic style was very much out of vogue but he refused to adopt the ‘neo-baroque’ or ‘Queen Anne’ like most other architects. He did however deploy a neo-Byzantine style for the synagogue on Adelaide Road (1892) and was comfortable using neo-Venetian elements for some clients such as banks and All Hallows College, Dublin (1884–7) in a style reminiscent of Deane & Woodward. His last major work was St Finian's diocesan college, Mullingar (begun 1902) which was completed after his death.
Though his later work was described as old-fashioned in some Dublin trade journals, one obituarist declared that he was ‘unquestionably one of the foremost Irish architects of modern times’ (Ir. Builder, xlvii, 918). He was certainly one of the better church builders in Ireland in the period 1875–1900, but never quite achieved the reputation of his rival J. J. McCarthy (qv); the ecclesiastical works of the two men are sometimes confused with each other. O'Callaghan's known output is considerable and he was able to turn his hand to anything from a large convent school to a modest tailor's shop. About a dozen public houses surviving in Dublin are attributed to him.
He was an active member of various architectural organisations (he was fellow, auditor, and council member of the Royal Institute of the Architects of Ireland) and made a significant contribution to the education and training of fellow Irish architects. As first president of the Architectural Association of Ireland in 1872 he gave a lecture on ‘the education of an architect’ and later delivered papers on the need for students to pay closer attention to measuring and studying ancient buildings. He lived at various addresses in and around Dublin including 3 Madeley Terrace, 21 Cambridge Road, 5 Alma Terrace, and 17 Alma Road. He suffered ill-health towards the end of his life and died 5 November 1905. He and his wife Eva had two daughters and six sons; two of his sons, Lucius (qv) and Bernard, became architects.