His theatre work has included Joe Penhall's The Bullet at the Donmar Warehouse, and a long-term collaboration with the Tony-nominated director Wilson Milam, including Ché Walker's Flesh Wound at the Royal Court Theatre and two critically acclaimed productions of Sam Shepard's plays: A Lie of the Mind at the Donmar Warehouse and True West at the Bristol Old Vic.
Tiernan played Piers Gaveston in Derek Jarman's controversial film of Christopher Marlowe's Edward II in 1991, after appearing in Lynda La Plante's award-winning drama Prime Suspect.
In the same year, he went on to star as Orlando and Oliver in Christine Edzard's version of Shakespeare's As You Like It playing alongside actors James Fox and Cyril Cusack.
He worked with Antonia Bird on a number of improvisational film productions, including Safe (Bafta - Best single drama), Face, Rehab and Spooks.
In 2005, he played Ben Jonson in A Waste of Shame, a William Shakespeare biopic presented as part of the BBC's ShakespeaRe-Told series.