[2] Costigan made her screen debut as crime figure John Traynor's girlfriend in 2003's Veronica Guerin, appearing opposite Cate Blanchett.
[8][9] In 2016 Costigan made her New York stage debut in Honor Molloy's Crackskull Row at the Irish Repertory Theatre.
[13] Later that year Costigan joined the original Broadway company of Jez Butterworth’s The Ferryman, understudying the roles Caitlin and Mary Carney.
[14] In 2019 she appeared opposite her mother Maria McDermottroe in Michael Scott's adaptation of The Valley of the Squinting Windows at the Mullingar Arts Centre.
[15] Later that year she starred in the critically acclaimed revival of Joe Crilly's On McQuillan's Hill at the Finborough Theatre in London.