Clarkin narrated The Gilbert Collection at Somerset House. He played Romeo and Prince Escalus in separate productions of "Romeo and Juliet", the first of which was at the Gate Theatre in Dublin and was directed and produced by Hilton Edwards and Micheál MacLiammóir. His other stage work includes Hamlet and Polonius in "Hamlet", Macbeth and Marcellus in "Macbeth", Salerio in "The Merchant of Venice", Thomas Becket in "Murder in the Cathedral", Eilif in "Mother Courage and Her Children", Roat in "Wait Until Dark", Estragon in "Waiting for Godot", in "Romeo and Juliet", King Henry II in "Becket", Ebenezer Scrooge in the musical "Scrooge", Henry VIII in "A Man for All Seasons", Bill Sikes in "Oliver!", First Voice Narrator in "Under Milkwood", Athos in "The Three Musketeers", Long John Silver in "The Secret of Treasure Island", Robert Folliett in "The Shaughraun", and the Spirit of Christmas to Come in "A Christmas Carol". He played William Burke in a Dylan Thomas adaptation of the story of Burke and Hare, "The Doctor and The Devils", directed by Roger Redfarn at the Theatre Royal, Plymouth. Clarkin also played Professor Hurst of the University of Padua opposite Keith Michell in the title role, in "The Artisans Angel", the story of Bernardino Ramazzini, directed by Keith Michell.