She is remembered for her English language performances at the Abbey Theatre in Dublin where she would eventually star in what many consider her finest role in the George Bernard Shaw play, Saint Joan.[2]

While performing at the Abbey Theatre in the 1940s, she met actor Denis O'Dea, whom she married in 1946.[3] Until 1970 they lived in Richmond Street South, Dublin. They had one child, a son: Donnacha O'Dea, who swam for Ireland at the 1968 Summer Olympics and later won a World Series of Poker bracelet in 1998.[2]

In 1955 she created the role of Miss Madrigal in Enid Bagnold's The Chalk Garden on Broadway.[4] In 1956, she appeared in the Cambridge Drama Festival production of Saint Joan at the Off-Broadway Phoenix Theatre. Theatre critic Elliot Norton called her performance the finest portrayal of Joan in memory.[citation needed] Siobhán McKenna's popularity earned her the cover of Life on 10 September 1956.[citation needed] In 1957, she joined the acting company of the Stratford Festival in Canada, playing Viola in Twelfth Night.[5] She received a second Tony Best Actress nomination for her role in the 1958 play, The Rope Dancers, in which she starred with Art Carney and Joan Blondell.[1] Although primarily a stage actress, McKenna appeared in a number of made-for-television films and dramas. She also appeared in several motion pictures such as King of Kings in 1961, as Virgin Mary. In 1964, she performed in Of Human Bondage and the following year in Doctor Zhivago.[1] She also appeared in The Last Days of Pompeii, as Fortunata, wife of Gaius (Laurence Olivier).

In 1979 McKenna appeared in the title role of Roald Dahl's "The Landlady", an episode of the British TV series Tales of The Unexpected.[6]

McKenna was awarded the Gold Medal of the Éire Society of Boston, for having "significantly fulfilled the ideals of the Éire Society, in particular, spreading awareness of the cultural achievements of the Irish people.