Gregg, Sir Cornelius Joseph (‘Con’) (1888–1959), civil servant, was born 26 January 1888 in Kilkenny city, eldest among three children of James Gregg, greengrocer and JP, and Marianne Gregg (née Gleeson). The family lived at Abbeyview and later High St., Kilkenny. Initially educated at CBS Kilkenny, Gregg later attended Blackrock College (1903–5) as a day pupil, where he won mathematical exhibitions in middle and senior grades. Following school he went to UCD and graduated with a BA in mathematical science from the NUI (1909). After graduation he was tutored in higher mathematics by Éamon de Valera (qv) before sitting the higher civil service examinations in London. In 1911 he entered Inland Revenue in London, where he was nicknamed ‘Corny’, a name he detested. He quickly made his mark as a civil servant of outstanding ability and industry and was appointed personal secretary to Hamar Greenwood (qv). Gregg agreed to a transfer to Dublin on the understanding that a new policy of conciliation was being adopted, but he soon found the climate of the Black and Tan era intolerable. He had decided to resign by early November 1920, and had done so by the end of the month, according to Mark Sturgis (qv), who found Gregg ‘a clever nice fellow but not a good private secretary and entirely out of sympathy with his chief’ (Michael Hopkinson (ed.), The last days of Dublin Castle (1999), 68, 82). Back in London Gregg's career prospered, and by 1922 he was assistant secretary of the board of inland revenue.
Between 3 April 1922 and 1 October 1924 Gregg was seconded to the newly established Irish Department of Finance, where he was the chief architect of the new Irish civil service. Appointed to the special rank of ‘secretary for establishment matters’, he used Whitehall as his blueprint for Merrion Street, only adapting it to suit the smaller scale. He also institutionalised the Whitehall mind-frame, most significantly the doctrine of financial control, thereby elevating the Department of Finance to a predominant position. Outside Finance he was largely responsible for the establishment of the office of the revenue commissioners and the civil service commission.
Gregg's civil service suited the state in the short term only, for after stability had been restored the Whitehall-type administration could not provide the radical change that the country required. Widely regarded as the ablest civil servant in Ireland, he expedited his departure because of what he perceived as the government's lack of loyalty to him. Returning to London, he became secretary of the board of inland revenue (1935), and later its deputy chairman, before succeeding Sir Gerald Canny as chairman (1942–8). During his period as chairman he oversaw a significant part of the turbulent war years, introduced the PAYE system of income taxation, and negotiated the double taxation agreement with the USA (1944–5).
After his retirement (June 1948) he returned to live in Ireland, where he was appointed chairman of the Irish board of the National Bank Ltd and remained a director of the company till 1956. His interest outside work was Thomist philosophy, and he was an active member of the Thomas Aquinas Society and the Thomas Moore Society. He was made CB (1932), KBE (1941), and KCB (1944) and received an hon. LLD from the NUI (1941). He died 14 November 1959 in Dublin.
He married (8 October 1917) Mary Anne, daughter of Thomas Crotty, baker, of Parliament St., Kilkenny city. They had two sons and three daughters, including Tom Gregg, sometime director of the National Rehabilitation Hospital, and Mary Gregg, UCD lecturer.