Looking fit, healthy and much younger than his 49 years, this Irvine Welsh is the successful, hard-working author you are more likely to find on a book tour or at a film festival than living it large at a club.

Prolific and driven, Welsh routinely produces more work in one year than most authors manage in a decade. In the 12 months since he released his fifth novel, The Bedroom Secrets of the Master Chefs, Welsh has completed a new collection of short stories (If You Liked School You'll Love Work), is about to finish another novel and, along with screenwriting partner Dean Cavanagh, is pursuing an increasingly successful career in film and television.

'I've got a bit lost in film lately,' Welsh says. 'Dean and I have been doing these scripts for years. The [early] ones weren't any good.

We thought they were great, but nobody else agreed. It's easy to take on the victim mentality, to say, 'They're w******, the world's against us'. In retrospect, they were probably right.'

Welsh estimates he and Cavanagh have about eight projects in varying stages of development, including adaptations of Alan Warner's novel The Man Who Walked, Welsh's novella Ecstasy and a new script about a Welsh male-voice choir. Welsh has also just directed his first short film, Nuts. 'You have to have three or four on the go because it's so hard to get even one feature made. It's surprising any films get made at all.'

This amiable, modest and business-like demeanour is light years from Welsh's popular image

as a 'depraved sleaze merchant' (to use his own phrase) - a shock tactician who delights in writing sordid tales of sex, drugs and bizarre bodily functions.

It's a reputation that owes much to Trainspotting, the stark, unsettling and frequently hilarious portrait of Scotland's junkies, psychos and schemers that made Welsh infamous more than a decade ago. He admits the book contains autobiographical elements: based on his own diaries, Welsh's 'hero' (Mark Renton) was born in working class Leith, drifted through various odd jobs, developed a drug problem, fell in love with rave culture and eventually escaped to Amsterdam.

But, he warns, don't be too quick to conflate song and singer. 'I was 33 when Trainspotting came out. A lot of that was about my 23-year-old self. But at 33, I couldn't recognise myself - I was always trying to find out what the f*** was going through my head then. Now that I'm pushing 50, I can't even see what I was like when I wrote Trainspotting. I have lived a whole different life again.'

Ask why readers so regularly confuse Welsh's life and fiction and he points to good old-fashioned British class prejudice. 'Critics never say Ian McEwan is like the f***ed-up stalker in Enduring Love,' he notes sardonically. These days, however, he prefers to see the muddle as paying a backhanded compliment to the visceral power of his prose. 'I spend a lot of time trying to get under the skin of my characters. I want their hot breath in your face. The more naive critics think that I somehow am all these messy, f***ed-up people like Renton and Begbie [from Trainspotting]. But I can't be close to them all. They're all so different.' What they will think of him when his next novel is released doesn't bear thinking about: Crime looks set to be his most controversial to date, about a mentally unbalanced Scottish cop who accidentally infiltrates a paedophile ring. 'It's very serious, very dark, but hopefully uplifting and positive. You can't ask anybody to read about paedophilia and make it unremitting. You need broader human elements just to make people able to look at it.'

Welsh denies that his being drawn towards the dark side of existence derives from any gratuitous desire to shock. These unflinching explorations serve his fundamental need for narrative drama and conflict. 'I look at people in extremis, in their bad six or 12 months. They're not necessarily

like that all the time. Renton's a heroin addict, but he's not going to be like that forever. Terrible things happen through stupidity, peer pressure or just bad luck. That's the crucible I put characters in to see what happens.'

Lots of terrible things occur in If You Liked School You'll Love Work. While these new stories don't hit the heights scaled by 1994's The Acid House, it does see Welsh broadening his horizons. Only one story, Kingdom of Fife, is set in Scotland. Another takes place in Spain, the remaining three being situated in Welsh's second home of America and boasting characters that include a cannibalistic cook and the world's most unlikely serial killer.

What links them, Welsh says, is people scraping the bottom of the existential barrel. 'When things are bad, they always seem able to get worse, often by decisions we make when we're not really thinking. By not rationalising, we compound the arbitrary cruelties of fate with wrong decisions.'

Nowhere is this clearer than in Rattlesnakes. One part 'farce in the desert' to two parts spaghetti western, this comic tale takes off when a young American is bitten in the genitals by a venomous snake. After the woman he adores refuses to suck out the poison, his male best friend is forced to do the phallic honours.

'My dad always said you discover who your best friend is when a rattlesnake bites you in the arse. It's really a story about everybody wanting something they can't have. One guy fancies this woman who fancies the other guy. They all have their own agenda, but nobody knows it overtly.'

For Welsh, this unrequited love triangle perfectly illustrates his sense of a broader dissatisfaction pervading modern life. 'There's tremendous pressures now. School kids forced to be young adults. Guys can't enjoy relationships because they're thinking of the next person to shag. We don't seem to have time to kick back and enjoy life. We have all these things on offer, but we're burdened by this mortality, this three score and ten. What do we do?' Welsh offers no answers, pointing out that his relentless work schedule has caused problems of its own. Far from settling down, married life has made him 'settle up' and live at a more frenetic pace than ever.

'I am flirting with the dangers. I live in Dublin most of time, and I've only been in the house five days out of the past two months. That's just ridiculous. You are heading for big problems.'

While Welsh is anxious that work and travel are ways of avoiding real life, he is just as concerned what he'll do if he ever does slow down. 'The devil makes work for idle hands,' he says. 'I do want to relax, but I actually fear what I would do with any free time. I no longer have the constitution to go out on the piss. It exerts too high a tax.'

Ageing does have its benefits. Welsh may not be the 24-hour party person he once was, but he has learned to cope with the ups and downs of his life. 'There's always the possibility of redemption because nothing good or bad lasts forever. As you get older, you learn to keep your head down in bad times until you come out the other side.'

Welsh pauses. And just when I think he has come over all optimistic, he delivers the sting in the tail. 'If you have good times, enjoy them because something bad is going to happen around the corner eventually.'

Writer's notes

Name Irvine Welsh

Genre Literary fiction, plays and screenplays

Latest book If You Liked School You'll Love Work (Jonathan Cape, 2007)

Age 49

Born Leith, Scotland

Current homes Dublin, Chicago and Florida

Family Married to Elizabeth. Previously married to Anne Ansty

Other works Trainspotting, The Acid House, Marabou Stork Nightmares, Ecstasy, Filth, Glue, Porno, The Bedroom Secrets of the Master Chefs

Other jobs TV repairman, worked in Edinburgh city council's housing department, property speculator, journalist

Next project Crime (a novel), screenplays including The Meat Trade (directed by Antonia Bird), a TV series based on one-off drama Wedding Belles, and an adaptation of his own novella, Ecstasy.

What the papers say

'One of the most significant writers in Britain. He writes with style, imagination, wit and force.'

- Nick Hornby, Times Literary Supplement

'Welsh writes the way Hogarth drew.' - Sunday Telegraph

Author's bookshelf

The Man Who Walked

by Alan Warner

Im converting it into a screenplay. Warners a poet. Sometimes you have to read his sentences or paragraphs about five times, and you get something new out of it every time.'

Moby-Dick by Herman Melville

It's one of the first books that I read. Its just such a fantastic story, it just grips you. It works on all sorts of different levels.'

Ulysses by James Joyce

'The first time I read it I didnt understand a word of it, but I forced my way through. The second time I thought, Not going to try to understand it. Im just going to see it as a trip through Dublin. A mad sort of chase.' It was like a gateway into all sorts of literature and thought.'

Lanark by Alistair Grey

'Its probably Scotlands Ulysses. When youve read it, you know you are never going to read anything else quite like it.'

Last Exit to Brooklyn

by Hubert Selby Jnr

'If I had written Trainspotting 10 years earlier it might have been a bad imitation of Last Exit. Its such a powerful book. It rang so true to me, someone growing up in a port town and a working-class scheme.'