Every writer wants to capture something of the human condition. Few manage it. But with his production of his own play, Irish dramatist Enda Walsh takes his audience as close to mortality as any new modern work I can think of. And not, as some have done, by writing so tediously about the subject that you want to end it all anyway. Rather he poses questions about his two tormented heroes who live together in a grubby, windowless room. And the answers are a little bit devastating.

We first encounter the duo, played by the sinewy Cillian Murphy and the blancmange-fleshed Mikel Murfi, in mid-ritual. The hi-fi is cued for ABC's 80s hit The Look of Love. To that optimistic pop song they launch into a ritualistic dance that audaciously lasts the length of the track. By the end of it, they have dressed, showered, breakfasted and talced. It is all part of a routine, as are the forensically rehearsed playlets in which they each perform, populated by the many denizens of a town called Ballyturk. Shopkeepers, loners, gossips - their sketched faces are scrawled on one wall, a map of the town's streets on another. These are scenes of disturbing hilarity punctuated by the kind of philosophical banter that brings to mind Godot's Estragon and Vladimir. And in its own way, Jamie Vartan's design of the bleak room, with kitchen cupboards bolted randomly on to the walls, a filthy shower in one corner and a kitchenette in the other, is the interior equivalent of Beckett's landscape.

As the non-events unfold, a question forms which, as it becomes more defined in the mind, changes the audience's perspective from that of voyeur. And the question is, do these lives belong to two strange men, or is it our own lives we are watching?

It is a notion that becomes ever more urgent with the arrival of a sinister, suited figure played by the chain-smoking Stephen Rea, whose progressively frightening presence begins to answer some of the questions posed. For all the absurdity here, there is much that chimes with our own lives, not least how the fear of dying is matched only by the terror of being left behind. The acting - delivering one of the funniest clownish double acts you are likely to find on stage - is flawless. Murphy is physically infected with malarial doubt that everything he has hitherto thought true is false. And although the play doesn't shed all of its mystery, it leaves you with a sense of desolate understanding, and the impression that you know more about your condition, and that of every other human, than is probably good for you.