Ó Ceallaigh, Cormac (1912–96), nuclear physicist, was born 29 July 1912 in Dublin, the only son among two children of Séamus Ó (Ua) Ceallaigh (James O'Kelly), consultant obstetrician and assistant master at the Coombe Hospital, and his wife Maire Cecilia (‘Birdie’) (née Ferran), daughter of a linen-factory owner from Magherafelt, Co. Derry. Séamus Ó Ceallaigh (James O'Kelly) (1879–1954), was born 27 October 1879 above the family pub at 25 John St., Belfast, eldest of five children of Michael (‘Micky Eoghan’) O'Kelly, originally from the Ballinascreen area, and Jane O'Kelly (née Bradley), also from south Co. Derry. His parents were distantly related and close links were maintained with home; summer holidays were often spent in the Ballinascreen region. Séamus developed a strong interest in the songs, genealogy, and traditions of the area. His father had great ambitions for his son and sent him to Clongowes Wood College, Co. Kildare. He then spent a year at QCB before moving to Dublin to study Irish and French. On receiving his BA (1902) from the RUI Séamus spent a year in France before proceeding to a medical career (MB 1909) at the Catholic University School of Medicine, Cecilia St., Dublin. After a period of time as the only anaesthetist at the Coombe Hospital, he was appointed assistant master (1917–20). Later, after spending some time in Vienna and receiving a Magister Artis Obstetriciae (master in obstetrics) degree on his return (1921), he set up his own obstetric practice (1920) from his new home in Upper Fitzwilliam St., Dublin. He was also doctor to St Enda's school, Rathfarnham, founded by Patrick Pearse (qv), and was an active member of the Gaelic League, as was his daughter Mór. From his student days he was also an old friend of Eoin MacNeill (qv). Although never involved in direct action, he made his home available as a safe house for the Irish Volunteers. On the eve of the Easter rising (1916), after organising a large meeting which included Pearse, Thomas MacDonagh (qv), and Joseph Plunkett (qv), MacNeill ended up issuing orders countermanding the rebellion from Ó Ceallaigh's home at 53 Rathgar Road. Ó Ceallaigh was called on by the rebels to treat the wounded in the GPO, and later treated anti-treatyites during the civil war. After getting wind of their plan to destroy the PRO in the Four Courts (30 June 1922), he pleaded for the sparing of the building. He wept in the street when the explosion went off, the only time he cried in public. Thus ended his direct involvement with politics. However, he continued his interest in the Irish language and Irish history; his waiting room held only magazines in Irish. As an amateur early Irish historian he collected the songs, history, and traditions of south Co. Derry and was a great authority on Derry and Antrim old Irish placenames (dinnseanchas). He helped Tadgh Ó Donnchadha (qv) (‘Torna’) prepare Cín-lae Ó Mealláin, as well as assisting others in their folklore publications; notably Paul Walsh's (qv) Flight of the earls (1916) and Henry Morris's (qv) Dhá chéad de cheoltaibh Uladh (1934). After translating two books, S. R. Keightly's (qv) The pikemen in 1936 and William Carleton's (qv) The black prophet in 1940, he published his own Gleanings from Ulster history (1951). He died 2 February 1954. His books and papers were acquired by the Irish placenames branch of the Ordnance Survey. He married (1911) Maire Cecilia (‘Birdie’) Ferran, whose family supported Séamus's medical studies after his own father ran into financial difficulties.
His son Cormac Ó Ceallaigh, named after Cormac na gclárseach (of the harps) of Ghleann Concadhain, Ballinascreen, was brought up as an Irish-speaker and became fluent in several languages. While his father was in Vienna (1920) he attended school at Mount St Benedict in Wexford. Although relatively lazy in secondary school, he applied himself to win an entrance scholarship to UCD (1930). Graduating with first-class honours in physics and chemistry (1933), he was awarded a postgraduate scholarship to carry out research (M.Sc., 1934). A subsequent postgraduate travelling scholarship brought him to the laboratory of physicist Pierre Auger at the University of Paris (1934–5), where he worked on cosmic radiation. Receiving another overseas research scholarship he spent a period (1935–7) at Trinity College, Cambridge, in the Cavendish laboratory under Lord Rutherford. Despite his own major scholarship he was awarded yet another, the 1851 (Commonwealth) Research Fellowship (1936). Rutherford, in a chance meeting on the stairs, announced: ‘I see you've wangled it again, O'Kelly’ (McCartney, 2002). He returned to Ireland to a lectureship in experimental physics at UCC (1937–53), where he continued his research and was awarded Ph.D. (1943). He had a reputation as a direct and insightful teacher, but after twelve years of a heavy teaching schedule he wanted to return to research. He applied for leave of absence (1949) and visited colleagues in Cambridge and Harwell before being offered a modest position in Bristol University by Nobel laureate Cecil F. Powell. His wife, Millie (a science graduate), was also offered employment. Despite making a considerable financial sacrifice to go to the UK, Ó Ceallaigh described it as a turning point in his life. It was a period of revolutionary discoveries in physics and Powell ran an innovative research group on elementary particle physics, reputedly one of the best laboratories in the world. Ó Ceallaigh became a leading figure in Bristol (1949–52). Here he carried out research on the unstable particles produced by cosmic rays using the nuclear emulsion technique, whereby thick photographic films were exposed to cosmic ray particles and the resulting tracks studied microscopically. These experiments were carried out at great heights, on mountain tops or from enormous high-flying balloons. He successfully identified a new meson, the K (kappa) meson or Kaon.
In 1951 he returned to his lectureship in Cork, and an additional position as research lecturer in nuclear physics was specially created for him. Some research funding was provided by the governing body of UCC and the following year (1952–3) by the DIAS. However, in 1953 he was appointed senior professor at DIAS, replacing Lajos Janossy (d. 1978). He held this post until his retirement in 1982 and built up an active emulsion group, continuing his association with Bristol and cooperating with a new particle research group in UCD. For a period his group used accelerators instead of cosmic rays as a source of particles. However, he returned to cosmic rays using novel techniques involving a new plastic material very sensitive to highly charged particles or heavy cosmic rays. These were carried on unmanned balloons and later satellites. Huge detector arrays were carried into orbit on the US space shuttle (1984), and remained in space for six years. The resulting data provided the most prolific information on the fluxes of the very heaviest nuclei. During his time at DIAS, he spent a year as visiting professor at Tata Institute of Fundamental Research, Bombay (1966–7), and periods at the research and development centre, General Electric Company, Schenectady, NY, USA.
He published over forty papers in international journals on several topics including nuclear and high-energy nuclear physics, cosmic radiation, and elementary particles. During his life he was a member of a number of institutes and acted on several committees: elected MRIA (1951), he was an RIA council member (1955–9), council member of the European Physical Society, member and chairman of the Irish National Committee for physics, member of Physics III committee at CERN, Geneva, fellow of the Institute of Physics, and member of Euratom scientific and technical committee. He received an honorary D.Sc. (QUB, 1975) and was winner of the RDS Boyle medal (1979).
A brilliant speaker, he had an ebullient temper and a mordant wit. Contrary to his flamboyant personality, his work was meticulous in the extreme. Despite his international standing and participation in international collaborations, he was given minimal recognition within Ireland. This is said to have fuelled his paranoia and he refused to take the Irish scientific scene seriously, gaining a reputation for impossibility. One of his ambitions was the establishment of an acceptable working climate for young Irish scientists. Although he received the RDS Boyle medal in recognition for his work, he refused to deliver a paper for the record. He did, however, join the group of nominators of future Boyle medallists and was responsible for the nomination of the 1981 medallist, statistician Roy C. Geary (qv). During the 1950s he was asked to advise the Irish government about accepting a research-scale nuclear reactor from the US government. He advised against, on the grounds that a supporting research group would cost more than the total of state research funding.
He married (1939) Millie Carr, teacher, whom he had met in Cambridge; they had three daughters. Outside his work his meticulousness was again evident in his skilled cabinet-making, one of his great interests apart from his family and physics. Sailing was his other passion. As a member of a number of Irish yachting and racing clubs, ‘he was known as “The Voice”, because in full flight he could be heard all over the coal harbour in Dun Laoghaire’ (Ir. Times, 18 Nov. 1996). He and his wife Millie raced in different classes of boat to avoid direct competition with each other. She predeceased him in 1987. He died 10 October 1996 at home (46 Killiney Road, Killiney, Co. Dublin), aged 84 years. In his honour the Ó Ceallaigh medal for distinguished contributions to cosmic physics was established through an association of the Ó Ceallaigh family, the Cosmic Ray Commission of the International Union of Pure and Applied Physics, and DIAS.