Actress. Born Siobhán Giollamhuire Nic Cionnaith in Belfast, Northern Ireland, the daughter of Mhaghréad O'Reilly and Eoghan Mac Cionnaith, a mathematics professor. She grew up in a Gaelic speaking household in Galway, English was her second language. She made her stage debut at the Galway Gaelic Theatre in 1940. In 1943, she became a member of Dublin's Abbey Theater troupe, and performed in both English and Gaelic. She met and married fellow actor, Dennis O'Dea, while in Dublin. She made her London debut in 1947 in "The White Steed." She made her Broadway debut in 1955 in “The Chalk Garden,” for which she received a Tony Award nomination for Best Actress. In 1956, her performance in “Saint Joan” earned a critic's assessment that her performance the best portrayal of Joan in memory. Her notices earned her the cover of Life magazine. She received a second Tony nomination for Best Actress in “The Rope Dancers” in 1958. In 1959, she made a notable appearance on American television in a "Playhouse 90" production of Shaw's "Misalliance." She reprised her role in “Saint Joan” for television in 1958 and “The Rope Dancers" on “Play of the Week” in 1960. She resisted feature films, but eventually appeared in several notable films including “King of Kings” (1961), “Of Human Bondage” (1964), and “Doctor Zhivago” (1965). She returned to the stage in 1971 with "Here Are Ladies," a one woman show. She appeared in the British anthology series, "Tales of the Unexpected," in 1979 and 1981 as well as the BBC Playhouse production of “The Journal of Bridget Hitler” in 1981. Her final film appearance was in the BBC production of “Angels in the Annexe” in 1984; her final stage appearance was in 1985 in “Bailegangaire” with the Druid Theatre Company in Galway. She succumbed to lung cancer the following year.