Brennan made her film debut in Attracta (1983), an Irish film that starred Wendy Hiller. In 2002, she appeared in the television remake of The Magnificent Ambersons. She played Hannah Arnold in the 2003 television film Benedict Arnold: A Question of Honor. Also in 2003, she appeared in Veronica Guerrin, an Irish drama film about the murder of Veronica Guerin. She also had supporting roles in Perrier's Bounty, Death of a Superhero, and John Crowley's Intermission.
For her performance as Mary Lacey, the mother of Saoirse Ronan's character in Brooklyn, she won Best Supporting Actress at the 13th Irish Film & Television Awards.
