A former hairdresser, Gallagher's big break came when she starred as Bernie in The Commitments (1991). She followed up with small parts in Pulp Fiction (1994) and Star Wars: Episode I – The Phantom Menace (1999). Other film appearances include You, Me & Marley (1992), Mary Reilly (1996), Divorcing Jack (1998), Thunderpants (2002), Skagerrak (2003), Tristan & Isolde (2006), Last Chance Harvey (2008), Sherlock Holmes (2009) and Grabbers (2012).