Costigan made her professional stage debut as a street child in the 1994 revival of
James Plunkett's
The Risen People at
The Gaiety Theatre. Staged as part of the
Dublin Theatre Festival, the production was co-directed by brothers
Peter and
Jim Sheridan, marking their professional reunion. At the time,
The Risen People was the most expensive event in the festival's 35-year history. She made her screen debut as crime figure
John Traynor's girlfriend in 2003's
Veronica Guerin, appearing opposite
Cate Blanchett. She would go on to have supporting roles in numerous Irish films, including
The Front Line (2006),
Becoming Jane (2007),
Swansong: Story of Occi Byrne (2009),
A Terrible Beauty (2013),
Halal Daddy (2017),
My Sailor, My Love (2022), and
TWIG (2024). Costigan has twice appeared as Mailí in
Aisling Ghéar Theatre Company's National Tour of
An Triail by
Máiréad Ní Ghráda, first in 2009, then again in 2013. The play is written and performed entirely in the
Irish language. In 2011 Costigan starred as Sandra in
The New Theatre, Dublin's production of
Isobel Mahon's
Billy the Boat Loves Angelina. The play tells the story of three siblings, Susan, Sandra, and Tim, who have become caught up in the
Dun Laoghaire drug scene. In a review for
The Irish Times, Peter Crawley noted Costigan's "assured performance", while "portraying (a) menacing figure with admirable bite." In 2016 Costigan made her
Off-Broadway debut in
Honor Molloy's
Crackskull Row at the
Irish Repertory Theatre. The following year, she appeared in
The Suitcase Under the Bed, a combined bill of four plays by
Teresa Deevy at the
Mint Theater Company. Both productions were named "Critic's Picks" by
The New York Times. In 2018 starred opposite
Hayley Mills in
Isobel Mahon's
Party Face at
New York City Center. For their respective performances, Costigan and Mills were both nominated for Best Actress at the 2018 1st Irish Awards. Later that year Costigan joined the original Broadway company of
Jez Butterworth's
The Ferryman, understudying the roles Caitlin and Mary Carney. The production would go on to win the 2019
Tony Award for Best Play; that year, she also appeared alongside her mother in
Michael Scott's adaptation of
The Valley of the Squinting Windows at the
Mullingar Arts Centre and in the critically acclaimed revival of
Joe Crilly's ''On McQuillan's Hill
at the Finborough Theatre in London. She appeared in The Irish Echo'' "40 under 40," Class of 2019. In 2020, Costigan appeared as
Molly Bloom in
The Ulysses Project, a work of
digital theatre, shot entirely during the first lockdown of the
COVID-19 pandemic. A low-tech re-telling of
Ulysses by
James Joyce,
The Ulysses Project featured an ensemble of over 75 actors who filmed themselves on phone or webcam, while being directed via
WhatsApp. The cast included
Olwen Fouéré,
Shane MacGowan,
Paula Meehan, and
Barry Ward.
The Ulysses Project was later screened by the
Irish Film Institute, as part of
Bloomsday Centennial in 2022; that year, she appeared in the original Broadway company of
Martin McDonagh's
Hangmen, understudying the roles of Alice and Shirley. The production was nominated for the 2022
Tony Award for Best Play. In 2023, she appeared as Grace in
London Classic Theatre's UK tour of
Brian Friel's
Faith Healer. Select tour venues included
New Vic Theatre,
Theatr Clwyd,
Malvern Theatres,
The Lighthouse,
Theatre Royal,
Cambridge Arts Theatre,
Hawk's Well Theatre,
Millennium Forum,
Middlesbrough Theatre,
Lichfield Garrick Theatre,
Devonshire Park Theatre, and
Connaught Theatre. In addition to her work on stage, Costigan has had recurring roles on
Vikings,
Fair City, and
The Marvelous Mrs. Maisel. ==Acting credits==