THE homeless in the modern city present a peculiar paradox. On the one hand they are, by definition, obvious. Living on the streets they are on public display, and homelessness is, as well as everything else, an absence of privacy. On the other, though, they are also invisible. As individuals, they melt into the streetscape. For various reasons - familiarity, embarrassment, anguish - they cannot really be looked at. Their lives are for the vast majority, mysterious.

For the theatre, their plight is particularly perplexing, because one of its assumptions is that we believe what we see, that if something is presented to us with sufficient conviction, it will affect us. Yet here, on the theatre of the streets, are stark realities that we see but do not believe in strongly enough to feel or do anything very profound.

Jimmy Murphy's new play at the Peacock, A Picture Of Paradise, is an attempt to solve the paradox. It sets out to reveal the lives behind the images and to tell the stories which make people suffering individuals, rather than abstract problems.

But it is, in the end, more a demonstration of the problem than a solution. For what it shows is, above all, just how difficult it really is to present for the comfortable society that fills theatre seats a convincing image of how and why the homeless become what they are.

This is, unapologetically, a play about a social problem, and the impulse behind it is profoundly realistic. But the difficulty of finding a form within which to contain such an impulse is at least tacitly acknowledged in the opening moments of the play.

For since it was published by Faber last year (in the collection The Dazzling Dark) A Picture Of Paradise has been very radically revised. The published version has a family breaking in to squat in a flat in the first act, then thrown outside in the second. Its tone is that of a flat, documentary style narrative.

Now, in David Byrne's fine production, the play begins with the expulsion the mother screaming as mattresses and bags rain down from an unseen balcony above. It is an arresting theatrical image. And with the help of Barbara Bradshaw's superb set - a stylised enclosure of grim towers - Byrne tries to push the piece further and further towards the kind of metaphorical resonance that an effective piece of theatre needs. He makes the family into a kind of Adam and Eve after the expulsion from Paradise, with the added, grotesque twist that the Eden they are desperate to reenter is a grim kip in a junkie infested compound.

This is undoubtedly the right way to go, and the surreal effect of having a family's furniture - armchair, table, mattresses - set up around a desultory tree and a garden bench does much to sustain the theatrical life of the piece. The problem, however, is that the writing remains so relentlessly realistic that it does not marry with the style of the production. The dialogue and characterisation have been restructured, but not quite reimagined.

There are good things in the writing. Flashes of the savage wit that made Murphy's Brothers Of The Brush so exhilarating break through the gloom. The contrast between a city that can afford to put a clock that doesn't work in the Liffey and the reality of those for whom hope is a lottery scratch card or a desperate double at the bookie's, is powerfully drawn.

And one of the strengths of the play is Murphy's refusal, for the most part, to wallow in simplistic sympathy. There is a proper political anger in his writing, but no glib suggestion that everything would be all right if the family had more money. The depiction of the working class is honest enough to include the way Angela (Barbara Brennan) gambles the savings away, the way his old friends turn away from Sean (Paul Bennett) when he is in trouble, and the fact their tormentor is a baseball wielding member of the flats' residents' committee.

But decency and honesty don't of themselves make for good drama, and there is simply not enough substance to make them live on stage. Realism is just not up to the job of dramatising fractured lives and broken worlds. Oddly, the more realistic the play becomes, the less convincing it seems. As characters too small scale for the elemental story in which they are caught, these ordinary decent people stuck in a hopeless situation come to seem almost prim and proper.

GOOD political intentions are wasted on thin theatrical forms. Conversely, there is no necessary contradiction between form and content - it is possible for a play to be both an exploration of theatre and an exploration of society. A play such as Frank McGuinness's oblique, highly charged monologue Baglady, for instance, is not just a better play but a more effective exploration of homelessness. And it is also worth remembering that the most memorable, most truly realistic theatrical image of what it means to be homeless, King Lear on the heath, is also the maddest and strangest sequence in theatrical history.