Sadleir, Thomas Ulick (‘Tom’) (1882–1957), barrister-at-law, genealogist, registrar at the Office of Arms, and deputy Ulster King of Arms, was born 15 September 1882 at the Curragh camp, Co. Kildare, youngest among three sons (one of whom died in infancy) of the Rev. Franc Sadleir, chaplain to the forces, of Castleknock, Co. Dublin, and Philippa Elizabeth Sadleir (née Burke) of Marble Hill, Co. Galway, who married (6 January 1871) in St Philip's church, Milltown, Dublin. Sadleir was educated at Dover College, Kent; Rossall School, Lancashire (1895–9); TCD (MA 1904); and the King's Inns, Dublin. His career as a barrister began when he was called to the bar in 1906, serving on the Leinster circuit (1906–16), but it was as a genealogist and researcher that Sadleir devoted his life and became most distinguished.
Having contributed in a voluntary capacity to research and compiling pedigrees at the Office of Arms, Dublin castle (1902–13), Sadleir was formally appointed to the office staff in 1913 as ‘understudy’ (1913–15) to the registrar, G. D. Burtchaell. He succeeded as registrar (1915–22), then as deputy Ulster King of Arms (1922–40), and following the death of Sir Nevile Wilkinson (the last Ulster King of Arms in Ireland) served as acting Ulster King of Arms (1940–43). In the latter capacity, he was the last civil servant in independent Ireland to be paid by the crown, as the Office of Arms (the Irish heraldic authority) was the last office of state to pass from British to Irish control (1943). After the transfer Sadleir opted not to accept the invitation of Éamon de Valera (qv) to continue working for the Office under the Irish government, and became assistant librarian at the King's Inns library (1944–57).
During his official career, Sadleir was responsible for tracing the genealogies and biographies of thousands of people of Irish descent throughout the world. He also published extensively on Irish family histories, architecture, and social history, with over 300 articles in books, pamphlets, and historical journals including Analecta Hibernica, the Journal of the Royal Society of Antiquaries of Ireland, Notes and Queries, the Journal of the Statistical and Social Inquiry Society of Ireland, the Journal of the Irish Memorials Association (which he edited 1916–31), and the Journal of the County Kildare Archaeological Society (which he edited 1922–47). As a member of the Irish Georgian Society he wrote Georgian mansions in Ireland (1915) with P. L. Dickinson. He edited An Irish peer on the continent (1920), and with G. D. Burtchaell two editions of Alumni Dublinenses (1924, 1935), a record of all the students who entered TCD up to 1846. Sadleir contributed the Irish research to Philip Guedalla's biography of the duke of Wellington (qv), The Duke (1931). As an inspector of the Irish Manuscripts Commission Sadleir surveyed and catalogued many private estate collections. He was a member of the Society of Genealogists (London), the RSAI, the RIA, the Athenaeum Club, London, and the Friendly Brothers and Kildare Street Clubs, Dublin.
His principal residences were 51 Lansdowne Road, Dublin; Temple Mills, Celbridge, Co. Kildare; the Old Rectory, Newcastle Lyons, Co. Kildare; and 57 Marlborough Road, Dublin. He married (29 August 1922) Anna Elizabeth Norman (‘Norma’), daughter of Dr James and Anna Kenny of Killeshandra, Co. Cavan; they had two sons, Randal and Digby. He died at Marlborough Road on 21 December 1957, and was buried at Castleknock church, Co. Dublin.