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).
An image of Gallagher, in character as Bernie McGloughlin in the film The Commitments, was featured on an Irish postage stamp as part of the Ireland 1996: Irish Cinema Centenary series issued by An Post. The image includes her The Commitments co-stars Angeline Ball as Imelda Quirke, Maria Doyle Kennedy as Natalie Murphy and Robert Arkins as Jimmy Rabbitte.
Gallagher's first album Precious Soul was released in 2004 on the Salty Dog Records label and was produced by John Reynolds. The album features collaborations with Brian Eno on the songs "He Don't Love You" and "Hooks". Gallagher wrote most of the music on the album, played the drums and sang lead vocals.
In 2009, Gallagher starred alongside a well-known cast on the MySpace/Vertigo Films production Faintheart and starred in the BBC Three comedy TV series Pramface in 2012.
