Ó Casaide, Séamus (CASSEDY, James) (1877–1943), bibliographer, was born 24 July 1877 in Fethard, Co. Tipperary, son of Michael Cassedy, RIC constable, and Honora Cassedy (née Manning). The family moved to Dublin, where Ó Casaide attended O'Connell Schools. He entered the civil service where he served in the Office of Public Works and later in the Department of Local Government. He continued to study while working and earned the degrees of BA, MA, and BL. He was known as ‘Séamus Ó Casaide’ by 1905 at the latest.
Ó Casaide took an active interest in many aspects of the Irish cultural revival. An enthusiastic rather than a skilful hurler and piper, he helped Éamonn Ceannt (qv) to establish Cumann na bPíobairí and was president of the body for a period. A member of the Gaelic League from about 1899, he took the initiative of establishing a library for the League's Ard-Chraobh, a task that contributed to his knowledge of Irish bibliography. A visit to the Co. Waterford Gaeltacht in 1902 left him with an enduring interest in the county's history and literature, an early fruit of which was his bibliographical study of Pious miscellany, by Tadhg Gaelach Ó Súilleabháin (qv), which appeared in Irisleabhar na Gaedhilge in 1903. He collaborated with Ernest Dix (qv) on a bibliography of books printed in Irish prior to 1820, which appeared in 1905. His later publications include A guide to old Waterford newspapers (1917); A typographical gazetteer of Ireland (1923), a study of the early days of printing in Irish towns; The Irish language in Belfast and County Down A.D. 1601–1850 (1930); and Watty Cox and his publications (1935). He contributed more than 400 articles, principally on bibliography and Irish-language writers, to a variety of journals – and especially to the Irish Book Lover, two volumes of which he edited.
Ó Casaide married (1918) Máire Ní Chonchubhair from Ráth Luirc, Co. Cork; they had two sons and three daughters. He died at his home, ‘Rathluirc’, St Lawrence Road, Clontarf, on 8 April 1943 and was interred in Glasnevin cemetery. He bequeathed his papers to the NLI.