Dagg, Thomas Sidney Charles (1875–1964), civil servant, hockey player, and hockey administrator, was born 25 December 1875, younger son of George William Jackson Dagg, barrister, of Airfield Terrace, Rathgar, Dublin, and Mary Dagg (née Kellett; d. 1877) of Dublin. Educated (1890–91) at High School, Dublin, he worked with John Jameson & Son, distillers, before joining the Congested Districts Board as a third-class clerk in September 1896. While with the civil service he attended TCD, where he graduated MA and LLB (1905) and was auditor (1906–7) and a gold medallist of the College Historical Society, editor of TCD (1904), and a distinguished hockey player, captaining Dublin University Hockey Club in 1905.
He played on the Irish hockey team twice (1903, 1911) and after leaving TCD joined Three Rock Rovers, where he was captain and president of the club. Called to the bar in Michaelmas term, 1909, he became assistant principal officer in the Department of Finance in November 1923 and principal officer in November 1931. After his hockey-playing career was over, he became the only official of the Irish Hockey Union (IHU) to serve as president (1920–24, 1930–31), honorary secretary (1907–8), honorary match secretary (1908–11), and hon. treasurer (1943–8). As an official he did more than any other individual to promote the game in Ireland and was responsible for the purchase of the IHU's headquarters and grounds at Londonbridge Road and of the Leinster branch's grounds at Templeogue. As a mark of gratitude for his service he was made the first patron of the IHU (1954).
He retired from the civil service in November 1940, though he was re-employed till April 1941. Dagg was deeply involved in the administration of the Royal Irish Academy of Music (RIAM), and in 1939 was appointed to sit on a committee to assess the position of local examination centres. In retirement he became more involved in the day-to-day running of the RIAM and was responsible for converting the band room into a concert hall. Opened 3 March 1945, it was named Dagg Hall in his honour. A highly active governor (from 1949) and vice-president (from 1964) of the RIAM, he was also president (1955–8) of the Strollers, a performance group founded in or around 1864.
Dagg's artistic interest extended beyond music and he was a gifted cartoonist, writer of doggerel, and punster, as well as a historian. His publications included Hockey in Ireland (1942), J. S. LeFanu 1814–73 (1949) (based on a lecture given in TCD chapel, 13 June 1949); ‘The Academy 1856–1956’ in The Royal Irish Academy of Music, 1856–1956 (1956); and The College Historical Society 1847–1920 (reprinted in part in Budd and Hinds, The Hist and Edmund Burke's club). Unmarried, he lived at 60 Waterloo Road, Dublin. He died 29 December 1964 in Cork, leaving estate valued at £9,121 to TCD for the foundation of a scholarship in history.