When Gary Mitchell was about nine years old, barricades went up at the end of his street and he was stopped from going to play with his friends. When he asked why, he was told that he was being protected from the Catholics.

"I was taught that if I ever met a Catholic, I should be wary, because they were going to be after me," he says. "And I couldn't understand it - why were they after me? I hadn't done anything. I could understand why Mrs Brown next door was after me, because I'd kicked a football into her garden. But what had I ever done to Catholics? I really thought the barricades were there to protect me specifically." That sense of threat, isolation and bewilderment is tangible throughout Mitchell's writing, and is the backdrop to As The Beast Sleeps, originally written as a play by Mitchell four years ago, and now adapted and made into a film by BBC Northern Ireland.

The film focuses on the impact of the ceasefire on one group of loyalist paramilitaries, and how loyalties - to family, friends and the loyalist cause - are tested to breaking point by the conflicting demands of the peace process. It is a dark vision and the performances are compelling. Stuart Graham plays Kyle, the leader of a UDA team who is charged with ensuring the new rules of the cease-fire are kept. His loyalty to the chain of command brings him into conflict with his wife Sandra (Laine McGaw) and best friend Freddie (Patrick O'Kane).

When the original play was written, and premiered in the Peacock Theatre in Dublin in a fine production directed by Conall Morrison, Mitchell's portrayal of individuals incapable of adapting to the new "rules" seemed bleak, at the very least. Now it seems prophetic, and his depiction of men tightly bound to the identity and structures provided by their membership of the UDA, is frighteningly accurate. There is a point in the film when one of the protagonists says: "Those days are over". But as we have all learnt since, those days are not over.

"The saddest thing is that four years after I wrote the play it is still very topical, and as relevant today," Mitchell comments.

At the end of the screening at Edinburgh International Film Festival last week there was absolute silence.

Watching this film is not an escape into the realms of fantasy. It is an intimate and uncomfortable glimpse of a real world and real people. At times it feels more like a documentary than a drama.

Mitchell adapted the original script for the film version of As The Beast Sleeps himself and has been closely involved in the production of the film. "When you have a stage play and it's successful the temptation is not to change anything," he says. "But it would not have made a good film - it's too long, there are too many speeches. It wasn't the kind of film I wanted to make or see. I wanted to see the film that Harry has made." The "Harry" Mitchell is referring to is film director Harry Bradbeer, whose credits include television dramas This Life and The Cops. Bradbeer confesses that he had no knowledge of Mitchell or his work before reading the script for As The Beast Sleeps.

"I liked the fact that it was a good story, the themes of loyalty and friendship are universal," Bradbeer says. "The characters and dilemmas are there - you'd have to work hard to mess it up. The dialogue didn't feel like writing, it felt very real. People can smell a lie and that's why it's important to get it right." Both Mitchell and Bradbeer agree that the fact that Bradbeer has no background in Northern Ireland was more of an advantage than a problem.

"Harry was the only guy who hadn't already made the film in his head by the time he got to talk to me," says Mitchell. "Having a guy from England, who isn't working class, who hasn't spent years of his life as unemployable wasn't an issue. If I had to explain something to Harry then I probably needed to explain it to the audience." Both men agreed that they did not want to glamorise violence. Bradbeer had seen some fights when he was researching The Cops, and had realised how clumsy, half-hearted and un-choreographed real life fights usually are.

"We are presenting violence as what it is - if it's too slick, it wouldn't be realistic," Bradbeer explains.

Mitchell's relationship with the BBC dates back to his first play, The World, The Flesh and the Devil, which won a Radio 4 competition and was first broadcast in 1991. To many audiences since then Mitchell has become the voice of working class Protestants in Northern Ireland, a voice which has all the more impact because it has been so rarely heard before.

"The community I come from don't see the arts as belonging to them," says Mitchell. "Working class Protestants think plays are silly things and can't change people's minds. When they come and see one of my plays they are always gob-smacked. They expect people to be running around in tights." Mitchell grew up in Rathcoole, a sprawling loyalist estate in north Belfast, and left school at 16 with few prospects.

The following eight years of unemployment made it all too clear to him how easy it was to get sucked into the paramilitary world. "It's as easy as getting up and putting on your clothes. There's an attitude that violence and threats work." Mitchell says.

Ultimately Mitchell's frustration found an outlet through a completely different route - he joined a drama group.

"I wanted to do a play where I could play me, and I'd be great, and everyone would love me," Mitchell says with a characteristic dose of self-deprecation. "When I couldn't find anything that represented me or my experiences I whinged about it so much the group told me to go and write my own. So I did." Mitchell tells a story of how when in 1998 he won the Irish Times/ESB Theatre Award for best new play for In A Little World of Our Own, the next day he was approached by a local woman in a garage. "She came up and said 'You're that wee fella Mitchell - didn't you win a wee award for a wee play?'"

Such reactions have clearly served to keep Mitchell's feet firmly rooted on the ground. He still lives in the community he grew up in and seems unperturbed by the idea that his writing and ideas might not be in line with his community's thinking. "Some people say 'why do you always have to show the bad side?' but the majority don't care." Mitchell regards his mission as getting his message out to as many people as possible and he is tickled by the idea that television takes his messages straight into people's homes.

"We'll see what the reaction is," he says. "I'm very proud of it.

"The underlying message is don't presume to know the difficulties of a country without ever visiting it," he says. "And secondly, violence never works. It only creates more violence.

"I do see the arts as a political movement. Theatre and cinema can change the way people think about things," Mitchell insists. "Obviously we're not going to put it on telly next Thursday and the peace process will click into gear. But it might be that someone says 'I've never thought about that before'. Or 'I've never seen that before' or 'I've never seen those people before'."

As The Beast Sleeps will be screened at the Belfast Festival in September and will be broadcast on BBC2 in the autumn