Ó Laoghaire, Diarmuid (1915–2001), writer and lecturer on Celtic spirituality, and Irish-language enthusiast, was born 1 August 1915 in Dublin, one of three children of Michael O'Leary from Doneraile, Co. Cork, and Mary O'Leary (née Flood), from Co. Meath; his father was manager of McBirney's department store on Aston Quay, Dublin. Brought up in Glasnevin and educated at Holy Faith convent school and Belvedere College (where he acquired a lifelong interest in cricket), he joined the Jesuits on leaving school in 1933.
Under the influence of his Irish teacher at Belvedere, the layman Tadhg Ó Murchadha, Diarmuid's interest in the language developed. He took Celtic studies at UCD, gaining an MA (1939) for a thesis on ‘Eochair-sgiath an Aifrinn’, a text on the mass by the seventeenth-century priest Geoffrey Keating (qv). He was awarded the NUI travelling studentship in Celtic studies. Because of the war, the British Museum manuscripts had been moved to Aberystwyth, Wales; at the suggestion of Robin Flower (qv), it was there that Ó Laoghaire pursued further research. Ordained in 1948, in the 1950s he was responsible for the Jesuit students in Rathfarnham Castle, while engaged in research, writing, and work with the Irish-language community. Prefect of studies at Belvedere (1960–62), he taught Irish at Gonzaga (1962–77), and thereafter was a member of the Jesuit community, Milltown Park. He was awarded an NUI Ph.D. from UCD in 1967 for a thesis on the lives of the saints, in Irish, in the medieval period. This research led to scholarly publications in Celtica, xxi (1990), 487–522, and a critical edition of ‘The Liber Flavus Fergusiorum infancy narrative’ (M. McNamara et al., Apocrypha Hiberniae (Turnhout, 2001), 142–245).
As well as speaking immaculate Irish, and fluent in French and Breton, he was well known throughout Wales, for he talked regularly on Welsh radio, and appeared on Welsh television. He translated a collection of short stories from the best modern Welsh authors into Irish (Glór ár nGaolta: Rogha scéalta na linne seo ón mBreatnais (1992)). For his abilities in Welsh he was made a member of the Gorsedd of bards in the Eisteddfod, the Welsh cultural festival; he preached in Welsh on occasions. His wide knowledge both of the spiritual texts and of the history and contemporary situation of the Celtic languages made him a respected authority on the Christian heritage of the Celtic world. On this topic he lectured in Milltown Institute of Philosophy and Theology, Dublin, and elsewhere. His Milltown Institute colleagues honoured him with a Festschrift (Mac Conmara agus Ní Thiarnaigh (eag.), Cothú an Dúchais (1997)), which included contributions from scholars in Wales, France, and Ireland. His academic, linguistic, and cultural interests were deeply integrated into his personal faith and his sense of mission as member of an apostolic order.
He was dedicated to exploring and fostering the link between Christian faith and Gaelic culture. Along with his more strictly scholarly interests, he devoted much time and energy to supporting and enriching the faith of the Irish-speaking community. This project was greatly energised by the change from Latin to the vernacular in the liturgy of the catholic church after the second Vatican council (1962–5). He rendered long and faithful service to a wide variety of groups, including Cumann na Sagart, Conradh na Gaeilge, An tOireachtas, Pobal an Aifrinn, An Chuallacht, Scoil Ghaelach Bhrí Chualann, and especially An Réalt, the Irish-language section of the Legion of Mary. In recognition of his services to Irish-language groups he was awarded Gradam an Phiarsaigh (the Pearse award) in 1992.
As editor of FÁS (Foilseacháin Ábhair Spioradálta, publisher of religious material in Irish), and through translations and other writings, Ó Laoghaire was one of those who ensured that a relatively varied spiritual and liturgical literature is available in Irish. Long a contributor, he was editor of An Timire (1972–97), the Irish-language devotional magazine founded by the Jesuits in 1911. A major contribution to the study of popular spirituality was his collection of prayers from the Gaelic oral tradition of Ireland and Scotland, Ár bPaidreacha Dúchais (1975), a book which has run into four editions. He died 21 July 2001 in Dublin. A catalogued bibliography of his books and pamphlets is in Milltown Park Library, Dublin.