Her career began when, as a young woman, she was recruited by the legendary Irish impresarios and theatrical directors Micheál MacLiammóir and Hilton Edwards.
She later married stage director Colm O'Kelly, who died not long afterwards of polio, which he contracted after swimming in the Nile during a theatre tour of Egypt.
[4] In 1957, she played Serafina in the first Irish production of Tennessee Williams' The Rose Tattoo and achieved unexpected notoriety when she and several other members of the cast were arrested for the possession of a condom on stage.
[5] Manahan played a minor role in the Irish cult soap opera The Riordans (1960s), and as Mrs. Mary Kenefick in the TV comedy Me Mammy (1970s).
She won the Tony Award for Best Featured Actress in a Play for her role as Mag in Martin McDonagh's The Beauty Queen of Leenane on Broadway.
In 2005 she starred in Sisters, a new play by Declan Hassett that was also written for her and for which she was nominated for a Drama Desk Award in the category of Outstanding Solo Performance.
She appeared in films starring, among others, Laurence Olivier, Peter Cushing, Kenneth More, Christopher Walken, Maggie Smith, Albert Finney and Brenda Fricker, and with John Gielgud in A Portrait of the Artist as a Young Man (1977).