Gallagher started her professional life as a hairdresser. She made her acting debut as 'Anne Conlon' in the 1989 TV movie 'Dear Sarah.' She simultaneously made appearances in films such as 'Screenplay' (as 'Frances'), 'The Bill' ('Nicola Purdy'/'Tina Wilson'), and 'Screen Two' ('Sandra Williamson').
Her first prominent role was in the 1991 musical comedy–drama film 'The Commitments,' adapted from Roddy Doyle's 1987 novel of the same name. She played the role of 'Bernie McGloughlin,' one of the three backup singers of the fictional band ’The Commitments.’
Her role as 'Bernie McGloughlin' became so popular that she, along with co-stars Angeline Ball, Maria Doyle Kennedy, and Robert Arkins, was featured on an Irish postage stamp, which was part of the 'Irish Cinema Centenary' series by 'An Post' in 1996.
In 1999, Gallagher made her stage debut as part of the tour of 'The Street of Crocodiles' (a collection of short stories by Bruno Schulz) produced by 'Théâtre de Complicité.'
She was seen in the brief role of Rosanna Arquette's (as 'Jody') on-screen friend, 'Trudi,' in the 1994 crime film 'Pulp Fiction.' Following this, she played 'Annie' in the 1996 horror film 'Mary Reilly.'
In 1998, Gallagher starred as 'Eileen' in 'Painted Angels' ('The Wicked Wicked West'). She then played the role of 'Carol' in the 1999 film 'This Year's Love.'
In 2000, she was seen as 'Teddy,' one of the main characters in the 'BBC Two' sitcom 'The Fitz.' Gallagher was cast as 'Miss Boyle' in the British comedy movie 'Wild About Harry,' released that year.
She played Bruce Cook's on-screen mother in the 2002 British–German–American family comedy 'Thunderpants.' The following year, she starred as 'Sophie' in the 2003 Danish drama film 'Skagerrak,' which was an official entry to the 25th 'Moscow International Film Festival.'
That year, she was also seen in the Ian Fitzgibbon-directed Irish movie 'Spin the Bottle.' In 2004, Gallagher entered the music industry with her first album, 'Precious Soul,' released under the label 'Salty Dog Records.' She composed most of the songs in the album. The album had her collaborate with English musician and record producer Brian Eno for a couple of songs. The album also featured Gallagher as a drummer and lead vocalist for a few songs.
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She played 'Bragnae' in the 2006 epic romantic drama 'Tristan & Isolde,' which depicted the medieval legend of ‘Tristan’ and ‘Isolde' and their love story.
Gallagher was seen as 'Sonya,' a member of a Christian group, in the 2007 horror–comedy 'Botched.' She played various characters in six episodes of the 'BBC Two' sketch-comedy show 'The Peter Serafinowicz Show.'
Gallager was next seen as 'Maggie' in the 2008 British comedy ‘Faintheart’ and as 'Hattie' in the 2009 British fantasy–adventure film 'Malice in Wonderland' (roughly based on Lewis Carroll's 'Alice's Adventures in Wonderland’).
She performed in 'The National Theatre' productions of 'War Horse' (as Rose Narracott) and 'Every Good Boy Deserve Favour' in 2009 (originally written by Tom Stoppard).
Following this, she was seen playing 'Eustacia' in the 2010 British romantic comedy 'Tamara Drewe,' which was based on the newspaper comic strip of the same name by Posy Simmonds.
That year, she played Janet McTeer's (as 'Hubert Page') on-screen wife, 'Cathleen Page,' in the 2011 British–Irish drama film 'Albert Nobbs,' an adaptation of a 1927 novella by George Moore.
Gallagher's first voice role was that of 'Elf' in the 2011 British–American 3D animated comedy film 'Arthur Christmas.' She also performed as 'Tatiana' in the 'Royal Court Theatre' production of 'The Faith Machine.'
She was also part of the documentary play 'Seven,' a 'Kabosh Theatre Company' production, in which she portrayed Mukhtar Mai, a real-life gang-rape survivor and women's education promoter from Pakistan, who was also the winner of the 2006 'Global Leadership Award.’
From 2011 to 2013, Gallagher played 'Trisha Meehan,' one of the main characters in the British crime drama 'The Field of Blood.' She also made a brief appearance as 'Una Maher' in the 2012 British–Irish monster film 'Grabbers.'
She starred in the 2012 American drama film 'After the Triumph of Your Birth.' From 2012 to 2014, she played the recurring role of 'Sandra Prince'/'Janet Prince' in 17 episodes of the 'BBC Three' comedy–drama 'Pramface.'
In the 2014 Irish film 'Shooting for Socrates,' she played the role of 'Irene.' She was seen as the recurring character 'Larrson' in seven episodes of the British–American comedy–drama 'You, Me, and the Apocalypse,' aired on ‘NBC’ the following year.
Gallagher was seen as 'Rachel' in the 2017 German drama film 'Return to Montauk' ('Rückkehr nach Montauk'), which was a 'Golden Bear' nominee at the 67th 'Berlin International Film Festival.'
She portrayed French art dealer Berthe Weill in an episode of the 'National Geographic' anthology 'Genius.' Gallagher then played 'Charlotte Stimple' in the 2018 historical romantic-drama film 'The Guernsey Literary and Potato Peel Pie Society,' adapted from the 2008 novel of the same name.
She appeared as 'Mrs. Burke' in the London premiere of the musical 'Girl from the North Country,' featuring songs of Bob Dylan, at 'The Old Vic.’ The musical ran from July 8 to October 7, 2017, and was then transferred to London's ‘West End’ at the 'Noël Coward Theatre' on December 29, 2017.
She has made brief appearances in 'Star Wars: Episode I – The Phantom Menace' (as 'Captain Maoi Madakor'), 'You, Me & Marley,' 'Divorcing Jack,' 'Last Chance Harvey' (as 'Oonagh'), and ‘Sherlock Holmes’.