There are, I reckon, two people in the world not quite convinced that Conor McPherson is a theatrical genius. I am one of them. And the other is Conor McPherson. The first doesn’t matter. But the second does. Doubt is good in critics, but writers need, at some point, to trust in their own gift, to free themselves from the anxiety of having something to prove. If McPherson is to be the great playwright he so often threatens to be, he has to arrive at that point.

I recently saw McPherson's latest play, The Night Alive , at the Atlantic in New York, which is probably the most prestigious of the off-Broadway venues. It was very hard to get a ticket. Ben Brantley, the eloquent and hugely influential critic of the New York Times , had hung it with garlands of luxuriant praise: "extraordinary"; "ascends to a plane that can only be called transcendent"; "the redemption of great art".

This kind of praise has followed McPherson's plays on their transatlantic passages from London to New York. ( The Night Alive is a Donmar Warehouse production and has not yet been seen in Ireland, where McPherson no longer debuts new work.) It puts him in the most rarefied company among international dramatists.

Which may make it seem absurd to suggest that this is a playwright who could do with more self-belief.

But artistic self-belief is not the same as success or fame or praise. It does not even have very much to do with ego. It’s the quality of holding one’s nerve, of trusting one’s instincts, of not being afraid of apparent simplicity. It’s the painter not panicking and making that one mark too many on the canvas. It’s the composer not adding that extra trill. It’s the novelist who kills her darlings and cuts her prose to the bone. And this, to me, is the quality that McPherson still has to find.

This matters only because he is very, very good. He has a tremendous gift for almost instant characterisation, which is why the best actors love to work with him. He has a great sense of theatrical form, albeit on a small scale: he writes chamber music, not symphonies or operas. He has access to a seemingly natural bitter humour, at once wry and hilarious. And he has the stamina that is essential to a playwright – already, in midcareer, he has a formidable body of work behind him. These gifts make him important in Irish theatre, the most obvious successor to Brian Friel and Tom Murphy. Except that he can’t quite seem to believe in them.

The Night Alive is perhaps the clearest expression yet of why McPherson is so good and why he struggles to achieve greatness. It is almost a wonderful play. But the bits that aren't wonderful are dreadful. This is something you don't see very often: there are lots of dreadful plays with good bits in them but very few really good plays with woeful elements.

All of the successes of The Night Alive come when not a lot is happening. The set-up is very simple. Shambolic, middle-aged Tony, separated from his wife and children, rents a room in a house near the Phoenix Park, in Dublin, from his widowed Uncle Maurice. The room also becomes a refuge for Tony's slow-witted pal Doc and a girl, Aimee, who has dabbled in drugs and prostitution. These fallen people are the essential elements of the play, which begins with Tommy entering the flat with a bloody-nosed Aimee, a stranger he has just rescued from her violent boyfriend.

In this core play nothing much happens, but it happens quite beautifully. These four people hover around each other, approaching and retreating, approaching and retreating. They nurture their griefs and grievances and hover around moments of tenderness or shared suffering. The writing is perfectly judged to leave just the right amount of space for a superb cast – Ciarán Hinds as Tommy, Jim Norton as Maurice, Michael McElhatton as Doc and Caoilfhionn Dunne as Aimee – to fill with vulnerability and brokenness and yearning.

The idea is simple – great loneliness in a crowded room – but it is enough to make an utterly absorbing little ballet of gestures and hesitations.

The problem is that, as well as this play, there is another one, and it is both lurid and ludicrous – the painter who has finished a beautifully delicate and muted miniature has lost his nerve and added a big red streak of vulgarity. It comes in the shape of a fifth character, Aimee’s boyfriend, Kenneth. He comes straight off the shelf at the Psychos “R” Us warehouse: a voice-hearing, hammer-wielding murderously malignant freak. He is, almost literally, the Devil. He batters Doc and is subsequently himself stabbed to death by Tony.

But – and this is where the whole thing becomes ludicrous – these actions have no real dramatic consequences. Chekhov famously ruled that if you introduce a loaded gun on stage, it has to go off at some point. Introducing a dead body that lacks a moral or dramatic consequence is a lot worse.

The pity is that this panic is a false alarm. The Night Alive is in no need of a random psycho to make it gripping. But until McPherson decides to do without devils he can't trust himself to make plays with his own best creations: ordinary people.

fotoole@irishtimes.com