Alan Gilsenan's new documentary offers an intimate portrait of Tom Murphy, one of the finest living English-language playwrights, writes FintanO'Toole

Alan Gilsenan's documentary on Tom Murphy, Sing on Forever, has an epigraph from the playwright himself: "Make the film as if I'm dead". In one sense, Gilsenan seems to have to have ignored that instruction. The pious process of embalming a dead man, wrapping his corpse in tight swathes of reverence, is certainly not what is going on here. The dark side of a complex personality is not filtered out. If he does not come to bury Murphy, Gilsenan does not entirely come to praise him either.

At a more profound level, however, this final programme in RTÉ's excellent Arts Lives series is made as though from a posthumous perspective. Murphy himself, though he is filmed singing with friends, walking in the west, working in Dublin and New York, speaks only in a disembodied voice-over, as if from beyond the grave. And those who know him best speak with an extraordinary frankness. The fear of giving offence or causing embarrassment to a living friend seems entirely absent.

The result is perhaps the most intimate portrait of a major living Irish artist yet committed to film. If it does not make the man ultimately less enigmatic, or pluck out the heart of his mystery, it does stand as a testament to the pain that has both surrounded and fuelled his achievements.

Of all living Irish artists, Murphy might be most easily forgiven for wanting to be indulged with a little uncomplicated celebration and earnest flattery. It was only in 2001, when he was 66, that the Abbey's fine season of six of his plays established that he ought now to be regarded as one of the half-dozen most important living English-language playwrights. Much of his previous career was marked by disregard and incomprehension. International acclaim has been slow and sporadic.

Born in Tuam in 1935, into a family torn apart by emigration, Murphy worked as a sugar factory operative and a metalwork teacher before he wrote, with the late Noel O'Donoghue, the brilliant one-act On the Outside in 1959. He followed it with one of the sensations of the early 1960s London theatre, the ferocious tragedy A Whistle in the Dark.

He was then the New Brendan Behan. Even the praise for A Whistle in London was loaded with assumptions about what an Irish playwright was meant to be like. The great Kenneth Tynan opened his review in the Observer with an extraordinary leap from the play to the personality of the playwright, whom he had never met: "Thomas Murphy is the kind of playwright one would hate to meet in a dark theatre. I have always been obscurely frightened of loudly singing Irishmen, and of Irish debaters who, corrugating their brows and stubbing my chest with an index finger, beg me to prove them wrong, and I am now convinced that I am scared of Irish dramatists as well. I have not met Mr Murphy but something whispers that he might unnerve me."

What was in fact genuinely unnerving about Murphy was that he was not a wild Irish broth of a boy, but a sophisticated European dramatist. The plays that followed A Whistle in the Dark - The Fooleen (afterwards called A Crucial Week in the Life of Grocer's Assistant) and The Morning After Optimism - are dream plays, dealing with the angular relationship between illusion and reality.

These were not the works of the new Brendan Behan. And because he was of course nothing of the sort, the London theatre lost interest in him. The immense influence of A Whistle on English drama (most obviously on Harold Pinter's The Homecoming) was scarcely acknowledged. There followed a long period of neglect, until the Abbey gave him a kind of home in 1968 when it produced Famine.

Back in Ireland after 1970, however, Murphy continued to evoke rather ambiguous responses. The Morning After Optimism was praised by most critics but encountered a great deal of bafflement among the audience. The Sanctuary Lamp provoked the most vocal disturbances in the Abbey since The Plough and the Stars in 1926. An RTÉ production of Murphy's The White House in 1977 provoked motions of protest from county and urban district councils condemning it as scandalous, scurrilous, obscene and filthy and earned a condemnatory editorial in the Cork Examiner.

The Blue Macushla was a box office disaster at the Abbey and was taken off after just a fortnight of its intended six-week run. Murphy's version of O'Flaherty's The Informer, even with a stunning performance by Liam Neeson in the main role, was greeted with overwhelmingly hostile reviews and box-office indifference.

It was not until the mid-1980s, when the Abbey's brilliant production of The Gigli Concert was followed by Druid's premières of Conversations on a Homecoming and Bailegangaire, that Murphy attained any kind of secure acceptance. Even then, it is easy to forget that The Gigli Concert, now acclaimed as a masterpiece, got many lukewarm reviews and did rather badly at the box office.

In the light of all of this, the temptation to make up for neglect with hagiography is strong. What makes Gilsenan's film so powerful, however, is that even while making the case for Murphy's stature in the theatre, it shows his achievement to be rooted in a restless imagination dogged by the black dog of depression.

Recalling his friendship with Murphy in Tuam, the distinguished theorist of science and technology Mike Cooley describes in the documentary their journeys on motorbikes, surveying the world like anthropologists in a strange land. He remembers one trip to the asylum in Ballinasloe and the haunting image of an inmate looking out. Alongside the warm memories of Murphy's sister Freda, and his Tuam friends Murt McCormack and Mary O'Donoghue, this image is a kind of ghost that hovers around the film.

Murphy's plays are marked by their deep sympathy with human beings on the edge of utter despair. In the testimony of his partner Jane Brennan, his wife Mary, the psychiatrist Ivor Browne and of Murphy himself, we get a sense of the deep connection between this sympathy and his own sufferings with depression. We also get a glimpse, especially in Mary Murphy's moving description of the break-up of their marriage, of the cost to others of the artistic struggle through which he has felt his way out of the darkness.

Yet the film is also haunted by the paradox that the depression that pains Murphy the man is also the force that powers Murphy the artist. In the hands of other writers the great soaring moments of transcendence that make works such as Bailegangaire and The Gigli Concert so extraordinary would be escapist fantasy. The reason they are so convincing is that they are so hard-won. They are pearls snatched from the depths at the very moment when the breath of life is about to run out. In illuminating those depths, Sing On Forever throws new light on a fierce struggle and a precious victory.

Sing on Forever is on RTE 1 tonight at 10.10pm