Published Wednesday, 23 July 2014
The Zero Dark Thirty star had been attached to star as a younger version of Vanessa Redgrave's lead character Roseanne in the project earlier this year, but she has since bowed out and now Mara will take over the role.
The film is based on Irish author Sebastian Barry's 2008 book of the same name, about a 100-year-old woman in a mental hospital who recounts her traumatic childhood in a secret memoir.
My Left Foot's Jim Sheridan will direct the movie, which will also star Jeremy Irons and Jack Reynor.
Production is due to begin in Ireland in September.