Brian Kennedy lets out a belch after several sit-ups and walks over to a mat where dumbbells await. He looks at them: "f**k". Welcome to Body Byrne, a studio off Grafton Street in Dublin where Paul Byrne puts people who are household names through gruelling exercise regimes that would make many gym bunnies cower in fear. He's the guy who got Keith Duffy looking like some kind of Corrie Hulk. He's the guy who gives Glenda Gilson her guns. He's the guy who crunches Rosanna Davison's abs of steel. If you're on TV in Ireland, and you need to bash yourself into shape, Byrne, it would appear, is your only man.

Back in the room, a TV3 presenter walks over to the water cooler for a sip before she's escorted by her trainer to the far side of the studio. Paul's wife Siobhán stops for a chat. Tall, striking and blonde, she is also a trainer at the studio. Her background is in martial arts. Before she knew Byrne, he was number one on her list of the best trainers around.

"He can look at a body and straightaway know what needs to be done, whether you need to be working on your outer thigh, or lower back." Byrne's intuition makes him one of the few personal trainers that chiropractors and physiotherapists refer patients to. Siobhán married him two weeks ago.

Elsewhere in the building, Grainne Seoige is working up a sweat. In the stairwell that leads up to the main studio room where Brian Kennedy is swearing at dumbbells, the walls are adorned with dozens of press cuttings of Byrne with his famous clients doing various stretches. Byrne himself is welcoming, a friendly sort who is so muscle-bound I find myself standing awkwardly against the wall so he doesn't notice my gut.

Byrne has been a personal trainer for 21 years. He got into fitness at the age of 10, first with karate. He was the junior Irish karate champion at 16, a black belt at 18, won the European karate championships five times over, and got a scholarship to develop his skills in Japan before a bad knee and retina injury he sustained in a fight in Thailand put a halt to his progression.

He returned to Ireland and focused on weight-training, in addition to competing in the Mr Ireland contest. One day, he went into the World Gym on Talbot Street in Dublin's north inner city with a friend who had signed him up. A few weeks later, the same friend came in and found Byrne running the place ? and telling him his membership was due. World Gym was old school (to the point that when two American marines arrived in one day to train, they thought it was a museum) and Byrne picked up tips from men who trained with Arnold Schwarzenegger.

"I started feeling in demand 12, 14 years ago. I wasn't doing too much. Back then, training people maybe six times a day would have been busy. I wasn't really set up." His first dedicated one-on-one training centre was in Temple Bar. He also did personal training at a Jackie Skelly gym.

He traces the celebrity end of his personal training, "I worked with Jim Sheridan and Daniel Day Lewis to film The Boxer, so how long ago was that? I trained Britney Spears when she was over here, Kerry Katona when she was Kerry McFadden. I'm lost on the years now, but I had big names coming in then."

His current studio is next door to a branch of Jackie Skelly. "Jackie, when she had breast cancer, she trained with me as well," Byrne says, "[we did] personal training to make sure she didn't damage herself. She's a good girl, a very strong mind."

But Byrne's flattery of his former colleague stops there. He doesn't like the gym chains. "All the big gyms, they're in the membership business, I'm in the fitness business. They're two different things... You'd go into any of the big gyms around Ireland, a lot of them would have five or six on their sales team, [and only] two instructors on the floor. The instructors mightn't have a clue what they're doing, probably just out of a course, paid €8 an hour and cleaning toilets and things like that. They're not really trainers."

He does have some praise for the bigger gym chains, "Westwood would be a good place. Ben Dunne has good-quality equipment. His concept is good. But if you go in there at peak time, you're queuing for a machine... He's filling the place with members, 11,000 in one of the gyms? It proves their whole concept of fitness is to get as many memberships sold and hope that a lot of them don't use it. Because if 11,000 members used the gyms then they couldn't possibly take them in. You'd need the Aviva Stadium to train them."

For Byrne, it's more than packing them in. First off, his studio isn't even open the public. Instead, everyone comes to him through referrals. "I keep it exclusive. I look after the clients that I have and I don't want to over-fill it... Joe Soap can't come up and want to book a session, it has to come from someone else. A lot of the clientele in here," he pauses. You can't have people gawking at them? I suggest. "Yeah. And nobody does, there's a great atmosphere, a great vibe."

He charges an average price of €65 to €70 an hour. He spends an hour, twice a week, with each client. That's all you need, he says. Any­one who charges €100 or €150 an hour for a personal training session is just trying to make themselves sound important, according to Byrne "A few years ago, there was just myself, Pat Henry – Pat's very good – and now there seems to be guys opening up in industrial estates with small studios, all looking at YouTube copying things."

Of course, all I want to know is, amongst his chain of celebrity hardasses, who's the ultimate work-out king or queen? "Eh," he hesitates, "I'm trying to keep confidentiality." Darn. I press him a little. "The hardest- working, without a shadow of a doubt, would be Grainne Seoige. Without a shadow of a doubt. Close second Glenda Gilson. Síle [Seoige] is up there as well. They work. They'd shame most people in a gym."

Byrne did a stint in Hollywood as well, training Jason McDonald for the film Sin City. His connections with agents prompted Britney Spears to seek a training session with him. Was she any good? "She was alright," he shrugs, "not a patch on Glenda or Grainne."

In his office, Byrne has a fridge stocked with pre-prepared meals showing he practises what he preaches. "My diet is pretty perfect."

He enjoys his weekends, and occasionally eats some junk food then (although I get the feeling his version of junk food is probably a little different to mine) but Monday to Friday is routine and he eats every three hours.

"For breakfast I eat eight egg whites and two whole eggs with some salmon. I come in here at 6am and do some cardio and light weight-training and then an hour later have some egg whites... Then I'll have grilled chicken and some sweet potato. Then I'll have some white fish and pasta. Then I'll have a protein shake before I head home. My evening meal is usually steak or fish or chicken with some green veg. I don't drink coffee, I only drink socially, I'm very anti-smoking." He shows me some pasta in Tupperware to illustrate.

Byrne repeatedly says he loves what he does. When he goes on holidays, he goes crazy if he can't find a place to work out, and all of his trainers – trained by him – have the same attitude.

"To survive as a personal trainer, you have to eat, drink, sleep and shit fitness," he states emphatically. "That's the only way you can motivate your clients. If it's just a job to you it's not going to work."

He gets up and goes back to the studio (not before imparting the advice that one pint of beer is 360 calories, which is one hour on the treadmill.) Later, he hands me a flyer for a charity night at Lillie's Bordello that Thursday. On Saturday, he'll be climbing Kilimanjaro with Kathryn Thomas and Síle Seoige, amongst others, in an initiative called 'Climb For Kids' to raise money for Crumlin Children's Hospital.

Grainne Seoige emerges and has a chat with Brian Kennedy. "I might see you Thursday night, Grainne," Byrne says as she's on her way out. "Super," she replies, as a credit union ad bearing her voice echoes through the studio's speakers that are fixed to Phantom FM. And after that, Byrne's back to work, sweat pumping, celeb training, pre- mountain climbing workouts. You'd hardly dare anyone to keep up with him.

Ab Fab: Paul Byrne's top fitness tips

Nutrition

»A proper portion of food is the size of your fist

»Eat five small meals a day. It keeps your metabolism burning without your body switching into starvation mode

»Drink loads of water

»Eliminate all sugary drinks from your diet

»The closer foods are to their natural state, the better they are for you

Training

»Do 30-40 minutes of cardio two-three times a week

»Do cardio sessions first thing in the morning on an empty stomach

»Lift weights. You won't bulk up, you'll just tone new muscle tissue. The more muscles you have, the more efficiently you will burn fat

Motivation

»Challenge yourself. When you feel like you can't do any more reps, do two more

»You can find time. There are no excuses

»It's all about mind over matter