Thompson, Sir William John (1861–1929), registrar-general of Ireland, was born at Tattyreagh, Co. Fermanagh, the son of William Thompson, a farmer of Tattyreagh. He was educated at TCD and the RCSI, graduating BA (1892), MB (1894), and MD (1895). In 1895 he was appointed house physician in St Vincent's Hospital, Dublin. Later he was elected demonstrator of anatomy at the RCSI and became a visiting physician and, eventually, a senior physician to Jervis Street Hospital. Having in 1905 been nominated physician-in-ordinary to the lord lieutenant, the earl of Aberdeen (qv), he became actively involved with Lady Aberdeen's (qv) Women's National Health Association of Ireland, most notably in the campaign to supply pure milk to children. The preeminent concerns of his long and distinguished medical career were infantile diseases and tuberculosis. He was a member of the committee for the 1907 Tuberculosis Exhibition, and his lecture on this occasion, entitled ‘Home treatment and nursing of pulmonary tuberculosis in Dublin’, was published in the second volume of Lady Aberdeen's Ireland's crusade against tuberculosis (3 vols, 1908). As registrar-general (1909–26) Thompson supervised the compilation of the 1911 and 1926 censuses. He also initiated the National Registration Act (1910) and the Registration of Business Names Act. During the First World War he oversaw the conversion of Dublin Castle to a military hospital and served as honorary treasurer of the Dublin Castle Red Cross Hospital, which provided some 350 beds.
Thompson held many public offices, including president of the RCPI (1925) and of the Statistical and Social Inquiry Society of Ireland (1918–20), consulting physician to the National Hospital for Consumptives, consulting 'medical visitor in lunacy' to the chancery court, member of the medical consultative board of the Royal Navy, examiner in medicine to the Navy Medical Service, and chairman of the committee of Peamount Sanatorium. He was both FRCPI and FRAMI, president of the Leinster branch of the British Medical Association, and vice-president of the Society for the Study of Inebriety. He received a knighthood in 1907.
A religious man, Thompson was very involved in the methodist church, and a few days before his death he delivered a speech to the general assembly of the presbyterian church. He was president of the Dublin Golfing Society and enjoyed travelling. On 9 September 1891 he married Mary Louise, daughter of James Wilson, a merchant of Dublin. Thompson died 9 June 1929 at his home, 59 Fitzwilliam Square, Dublin. A list of his articles (mainly on public health and statistical matters) can be found in Richard J. Hayes (ed.), Sources for the history of Irish civilization: articles in Irish periodicals, 9 vols (1970).