He claims he can make people thin, and forget their wife's name, but Rosita Boland resists Paul McKenna's spell.

The man who can make you think a broom is Cameron Diaz is occupying the presidential suite of the Westbury Hotel. Hypnotist Paul McKenna is watching the huge flat-screen television that comes with the suite. It's interesting to see what hotels think presidents need. There are a lot of large mirrors, a Christmas tree, several tables of varying sizes, a fireplace that looks fake, and a bar. There is a bottle of unopened champagne sitting on the counter (should it not be in the fridge?). Oddly, there is no private lift access, but perhaps this is why other presidential suites in this country tend to be a bit more famous.

Anyway, Paul McKenna (41) looks right at home here. His trainers are under the table and he's walking round the suite in his socks. He's not a president, but he is a millionaire. He made his money from television shows and from his hypnosis training company. He also produces self-help books, tapes, CDs and DVDs on topics such as how to give up smoking, sleep better, mend your broken heart and cope with stress. This year, Change Your Life in 7 Days was a self-help bestseller. A proof copy of his latest book and CD, I Can Make You Thin, is lying on the couch between us.

"I read it on the plane on the way over," McKenna says with enthusiasm. "I was trying to imagine I hadn't written it, and was there anything I would have done to make it better if I had written it." I spend a minute or two working this out. Then I wonder: do people usually read their own books on aircraft? Especially ones that have a large photograph of your face on the front cover? What does that say about you? I think it must mean your ego is robust.

McKenna, who is originally from Enfield in London, and now lives in Kensington, started off his working life as a DJ. He worked with local radio stations and then for Radio Caroline. "I went one day to interview a hypnotist and got really interested. He did it to people for relaxation. I borrowed a few books from the library and taught myself how to do it."

He makes it sound so easy, but how did he learn how to hypnotise people - including himself?

"I'm very interested in human beings," McKenna says brightly. Er, yes, but how do you learn how to be a hypnotist? "I'm a people person!" he answers. In other words, he's not telling us the secret of his millionaire success.

McKenna initially made his name though his popular comedy shows, both live and on TV. He still performs live, but says it's "really only a hobby now. I just came to Dublin this weekend for fun and to see my friends. I don't come to Ireland for commercial reasons." He performed two shows at Vicar Street this month; his routine show. He asks for 25 volunteers and then whittles them down to 10. How does he choose them?

"It's partly intuitive," he explains. "They way they hold themselves, the way they look at me, the way they respond to a couple of things I ask them to do. I can tell if someone is engaged or not; in hypnotism, you lead someone into an altered state."

The volunteers are hypnotised into believing things that are untrue are in fact true. "They ride imaginary horses. Or think they're Martians. Or they think a broom is a famous film star - I had one man lately in Grimsby who thought the broom was Cameron Diaz. He started trying to disappear into the wings, saying that he wanted to show Cameron his 'beast'.

"I can make men forget their wife's name. Dublin audiences are much less inhibited."

Can he hypnotise animals? "I've never tried. I don't think so. Hypnotism is language-based. They might catch your tone, but they wouldn't understand what you were saying." So if he had someone on stage who didn't understand English, he wouldn't be able to hypnotise them? "I wouldn't be able to reach their sub-conscious."

Apart from writing self-help books, McKenna has become well-known for his hypnotherapy techniques.

"It's the best technique to use for phobias and fears. I get rid of them in one session - what would take a psychotherapist six months. Analysis is f***ing rubbish. It doesn't work. It's like taking someone with a broken leg and continuing to throw them downstairs until they feel better.

"We're born with only two fears - the fear of sudden noise and the fear of falling. Everything else has been learned . . . I have no time for Freud," he adds, rather unnecessarily.

He hypnotises people so he can work on insomnia, confidence, fears, phobias and addictions. The big money is in the corporate sector, where he can facilitate workshops for several hundred people at one session. The ability to develop positive thinking and self-improvement is a large part of this kind of work. He also has private celebrity clients; among those listed on one of his websites as having been treated by him are Daryl Hannah, Ronan Keating, Gabrielle and Robbie Williams.

I'm puzzled. If you claim to get rid of a fear of claustrophobia in one session, what happens to the man you make forget his wife's name at a show? Do they both last the same length of time? Apparently not. They're different techniques.

"The man isn't going to forget his wife's name forever. The subtext in the show is that the volunteer is going to take part in this. It's like a karaoke session - you're going to sing because your inhibitions are lowered when you've been drinking."

The photographer arrives and McKenna disappears to change into something more formal for the shot. I have a look through I Can Make You Thin. There are quite a few eye-opening before-and-after photographs in the book of people who have come through a serious weight problem, although McKenna is no different from other get-thin authors by sanctioning the use of swimwear for the "after" pics. One of the accompanying case studies says: "Sometimes I think back to not even being able to wipe my own backside. I know that sounds disgusting, but that's how big I was! I've gone from 28 stone to 10 stone in two years."

After the photo shoot, McKenna explains his book is radically different to all other get-thin books.

"Everyone else is doing diet books. My book teaches people how to resensitive themselves. They don't need to exercise. They just learn how to eat less and to listen to their bodies."

Is this not self-control dressed up a different way? No, apparently not. At this point, I'm getting addled by McKenna's quite remarkable ability to deflect questions. Is it my imagination or am I losing my concentration?

I Can Make You Thin by Paul McKenna is published by Bantam Press in January