Stephen Nolan grabs stories with a terrier grip. With up to 2,000 callers, it's a powerful show, he tells Susan McKay

He's big, he's brash, he's boastful, he's very bloody annoying, and he's absolutely brilliant. He has won more Sony awards, the radio equivalent of the Oscars, than any other British broadcaster. He has also been named as one of the UK's "top ten arseholes" by Viz magazine. His theme song is a roisterous instrumental with shouts of "Hup! Hup! Hup!", and then Stephen Nolan invites listeners to his morning radio show on BBC Northern Ireland like a manic fairground salesman.

"Listen up now! This is the biggest show in the country! Here's what we do!" Here's what we do introduces a minute or so of soundbites taken from the previous day's show, and often consists mostly of people hurling abuse, gleefully urged on by Nolan. When he gets a really good row going on the show, he'll squeal with laughter and shout, "I'm going to wet myself!" When he tires of a particular melee, he's liable to say, "I've had enough of this. You can all get stuffed."

On one recent programme he got a call from a milkman who'd had to give up his round in the wealthy Malone Road area because people wouldn't pay him. Nolan had great fun. He championed the milkman and berated the "posh with their Bentleys" who'd need to "wise up to themselves".

"Cameron," he shouts, like a lord of the manor shouting for his butler. Cameron is the broadcast assistant. "Cameron! Get me some of these posh people on to explain themselves." Soon milkmen of several generations - including an old man who used to deliver by horse and cart - were calling. They said it had been always thus. The poor would pay, the rich would not. "You're the people's champion," one of them told Nolan.

Then the original milkman got carried away and made a mistake, appearing to let slip that he earned £4,000 a week. Nolan seized on this delightedly. "I'm going to give up the BBC and go and be a milkman - that's where the money is," he declared. A man called Stevie rang in berating the milkmen as chancers.

A milkman lost his temper. "Yon boy's nothing but a gance," he said, sourly, of Stevie. This prompted Nolan to bring the gance [one of Belfast's many terms for a ne'er do well] back in for another round. The original milkman kept trying intervene - he meant £4,000 per month and that was before he paid for the milk . . . but it was too late, the Nolan show had swept on by.

Slagging is as integral a part of the Belfast experience as rain, and Nolan is a master of the art. However, that's not how he got his name as "the people's champion". Take just one of last week's programmes. At its pre-show meeting, the Nolan team decides to abandon much of what it had intended to cover because one of the producers has been bowled over by the BBC's Panorama investigation into paedophiles the night before.

"I was astonished," Nolan tells his audience, describing how the programme showed convicted rapists befriending children, and abusers on probation hanging around school gates. He gets the North's chief probation officer, Brian McCaughey, on the line and keeps him there for an hour, taking calls from, among others, a young woman who'd been sexually abused as a child.

She denounces McCaughey because he has admitted he doesn't personally know at all times the exact whereabouts of the 10 known high-risk offenders who've been released from prison in the North and are under some form of supervision. McCaughey says one of his people would be in charge of managing 15 offenders, and valiantly tries to explain the complexities of the multi-disciplinary approach.

Behind the glass wall separating the studio from the production area, producer Mike Lee, has sympathy for McCaughey. "Fair play to you, Brian," he murmurs. After the show, producer Camilla Carroll will call to thank him. The production team is unfailingly polite and grateful to all who contribute to this white-knuckle ride of a show.

Nolan has his eye on the texts and calls coming in on his screen in the studio. "The thing you are really biting on," he tells the listeners, "is that there is only one professional managing 15 offenders." The programme breaks for the news at 10 o'clock. The first headline is that the chief probation officer has said on Nolan that he needs more resources to improve the supervision of sex offenders.

During the last half hour of the show, a woman phones in to reveal that social services have been sending young sex offenders to the hostel for homeless and vulnerable adolescents in which she works, without informing staff of their background. The hostel has a policy of not accepting sex offenders, for the protection of other children. "What in the name of God is happening?" says Nolan. He promises to find out. "What makes this show worthwhile," says Carroll, "is that sometimes we get to help people."

Nolan gets a hold of stories in a terrier grip, and won't let go. There was the case of the young woman from Co Down who had a terminal illness which left her too disabled to care for her newborn baby without help. When she left hospital, the baby was kept there. She said social services told her she'd have to make her own arrangements with a nanny or au pair or else they'd put him into foster care. Within a week, arrangements had been made and the baby was at home with his mother.

Now into its third year, The Nolan Show is listened to by those charged with minding the backs of government ministers, and they frequently feel compelled to issue defensive statements. He must be the bane of the lives of senior social work people and civil servants. Health boards routinely respond to his team's demands for answers with statements to the effect that they don't make statements about individual cases for reasons of confidentiality.

"For goodness sake," Nolan shouts on air. "Why do they say that when the parents have called us in the first place? Spit it out! My blood pressure is going through the roof."

Then it's time to hand over to Gerry Anderson in the Derry studios. The pair spar daily at this time. This time it is over which of them won a boxing match undertaken for the BBC's Children in Need appeal. Nolan bounces up and down in his seat and claps delightedly. "Geriatric!" he shouts. "Text him. Geriatric!" He laughs wildly. Anderson, whose suave and deadpan wit is a perfect foil for this boyish buffoonery, takes it away, and Nolan moves seamlessly into pre-recording an interview with Ben Elton for the next day's show.

Afterwards, we sit in the studio. It is Nolan's natural medium. He is lizard-eyed and alert and disarmingly frank, an endearing class of a man.

"I am ruthlessly ambitious," he says. "I fought and fought and spent years trying to get into this building. This was my dream. There were senior people in there said I'd never get a job here. I wrote at least 50 letters. I once left here in tears. Now I've got the biggest show in the country and I completely love it."

Despite his eight Sony awards and several others, he says the broadcasting business is too competitive to start getting complacent. "This show takes risks," he says. "I have to tread a very fine line. Anything we say, we have to deliver. The show is successful because it is real. We tap into the super-ordinary views of ordinary people. People trust us with their stories."

He fought to get a paramilitary on the show who had carried out punishment beatings, and got a huge response from listeners, as well as one of those Sony awards. "On a big day we get 2,000 callers," he says. "This is a very powerful show."

Now he is keen to persuade the BBC to let him interview a paedophile. "If you are going to have integrity, you have to hear from the protagonists." His mobile rings. "Hold on, it's my Mum," he says. His face softens and he smiles as he tells her he'll call her back. "Mum's just fantastic," he says. "She talks as much as me, she's as loud as me and she's as crazy as me. I'm skundered when she slaps me on the face in shops and says, "shut up you" and calls me 'this wee boy'. I'm 33 years old!"

All the calls he gets about people who have had miserable childhoods reinforce his sense of good fortune. "My parents loved every bit of me."

He's a working-class boy from the Ballygomartin Road (above the Shankill on the mountain slopes of west Belfast) and he reckons the BBC needs more like him. "Why should you have to wear a suit and never say anything surprising?" he asks. At his local primary school he had "tipped hair and rings on my fingers". Then his parents "dragged" him to grammar school. "They wore black socks there, which I thought was wrong, because I wore white socks," he says. "I played football, they played rugby." Soon, though, he loved it. "It changed my life and gave me confidence," he says.

He went to Queen's University, and started his broadcasting career at Belfast Community Radio which became City Beat. He quickly developed a talk show that attracted a huge share of Belfast's radio audience. "I now work with the back-up of an excellent team, " he says. "Back then I had to research, produce and present on my own. The BBC wonders why nothing makes me panic.

"Personality radio is where it is at," he says. "I'm unashamedly tabloid, but I'm not a shock jock. That's a cheap shot from people who don't like me asking the questions no one else has the balls to ask."

He flaunts his slobbishness. "I'm 20 stone and I had a doctor on the show last week said I was 'morbidly obese'," he says, with bizarre pride. "But I have the heart of an athlete and the blood pressure of someone who trains every day." He boasts on air that he has dozens of parking tickets and refuses to recycle his rubbish. "I'm an ordinary guy," he says. "I'm lazy."

Driven, though. As well as the Belfast show, he does three nights on the BBC's Five Live, which means a frantic commute to Manchester. He's in talks over a new series of his television show, Nolan Live, and he says he has lots of other offers. He's a big fish, torn between ambition and his love for the small pond that is Northern Ireland. "Here's what I do know," he says. "I won't be leaving my mother."