Brennan starred opposite Philip Glenister as Frances in Ronan Bennett's political conspiracy thriller Hidden, a role which saw her named Woman of the Year in the Arts by the Belfast Telegraph.

[2] In 2007, Brennan starred as Regine Engstrand in the world premiere of Henrik Ibsen's Ghosts,[5] in a new version by Frank McGuinness, directed by Robert Bowman at Bristol Old Vic.

[6][7] That same year, she also starred as Helen in the Irish premiere of Owen McCafferty's Scenes from the Big Picture, directed by Conall Morrison for Primecut Productions.

This Louise Ramsden adaptation is based on the blog The Secret Life of a Manic Depressive and is an account of learning to live with bipolar disorder.

[10] The year 2010 saw Brennan tread the boards of The Gate Theatre in Arthur Miller's masterpiece Death of a Salesman, directed by David Esbjornson with Harris Yulin playing Willy Loman.

[22] In 2017, she took on the lead role of Tania in the world premiere of Marie Jones' hit comedy Sinners at the Lyric Theatre, directed by Mick Gordon.

[13] In 2019, she starred as the American Consul's wife Anna McVeigh in the Netflix original drama Hache alongside Javier Rey and Adriana Ugarte.

Hache drew inspiration from true events, unfolding against the backdrop of the heroin trafficking business amidst a world of violence and allure.

Bitter Pill is an audio drama series following Mary's trauma after a dreadful car crash and her journey through a clinical drug trial that hopes to cure her PTSD.