chief justice of the Irish Supreme Court (1961–74) and fifth president of Ireland (1974–76).

Son of parents active in the struggle for Irish independence, O Dalaigh studied at University College, Dublin, earning a degree in Celtic studies in 1931. He was an Irish-language editor on The Irish Press (1931–42), meanwhile becoming a lawyer (1935). In 1945 he became Ireland's youngest attorney general, serving at that post in 1945–48 and 1951–53. In 1953 he became a judge on the Supreme Court and was elevated to chief justice in 1961.

When Erskine H. Childers died in 1974, O Dalaigh succeeded him as president of Ireland, but he resigned three years later in a clash with Defense Minister Patrick S. Donegan, who called him “a thundering disgrace” for using his presidential powers to delay legislation designed to thwart the terrorist activities of the Irish Republican Army; O Dalaigh considered the legislation unconstitutional.