Johnny Vegas touches an audience member. Sean Hughes makes a Madeleine McCann joke. Joan Rivers ridicules 9/11 widows. The comedy routines that scandalise some people will have others rolling in the aisles. As the Kilkenny Cat Laughs festival prepares to stage an evening of deliberately offensive material, Brian Boyd looks at comedy's dark side

THE DAY after the 7/7 London bombings, Australian comic Greg Fleet began his stand-up routine with an observation about London Transport. "It's always the same - you wait ages for a bus to come along and then two-and-a-half come together."

It was a line that divided the room. Was it offensive? After all, 52 people were killed and 700 injured in the bombings. But the joke had nothing to do with the victims - so, did it have permission to be funny?

Whenever three comedy taboos walk into a bar, controversy and hilarity will ensue in equal measures. From Lenny Bruce to Jerry Sadowitz, many comics have attempted to hurdle over the line separating humour and offensiveness. Is any subject matter truly taboo? Can race, gender, sexuality, disability ever be treated in a meaningfully comedic manner? A recent Channel 4 documentary promised to reveal "the world's most offensive joke". Hampered by the simple fact that there can be no absolute sense of offence (what offends you, I may well find hilarious), the programme presented its findings as a top-10 countdown.

In the list were the usual suspects - the Holocaust, natural disasters and Princess Diana jokes. The programme decided that the cartoons depicting the Islamic Prophet Muhammad which were published in a Danish newspaper constituted the most offensive joke in the world - simply because they offended so many people.

In recent years, instances of taking offence have greatly increased. South Park, Jerry Springer: The Opera, Little Britain and Ricky Gervais have all been accused of gratuitously offending someone or something. There are those who feel freedom of speech is merely being used as a convenient shield for those intent on being wilfully disrespectful. Others feel that a certain comedic robustness has been sacrificed on the altar of political correctness.

The above-mentioned Greg Fleet is one of the stars of The Dark Show at this year's Smithwick's Cat Laughs comedy festival in Kilkenny. Fleet will be joined by UK comic Andrew Lawrence and Irish comic Carol Tobin. The three are charged with exploring the darker side of comedy - and the show comes with a strict "if easily offended - stay away" proviso.

For Tobin, the only Irish comic on the bill, The Dark Show is a dream gig. "If there's ever a show that suits me, this is it," she says. "It's not that I do controversial or crude material, but my subject matter just happens to be full of dark, surreal imagery. I always dread when I'm first on a comedy gig because some people just don't know what to make of the material."

Tobin deals with subject matter that other comics simply don't touch. "It's just a frame of mind, really," she says. "I talk about bad thoughts I've had - stuff that usually people would never admit to in public, some of the stuff is quite morbid - it's the sort of thing you don't really expect to hear in a comedy club. It's not offensive . . . maybe it is to some people - but that's never the intent."

Tobin frequently gets concerned inquiries from audience members after she performs. "I get some people coming up to me afterwards and giving me a hug," she says. "Others say: 'Do you want to talk about it?', and I'm sure others are wondering about what sort of childhood I had. I once had a lady say to me that she really enjoyed the show but she wasn't able to laugh at anything I said. It was only when she got home, she said, that she could laugh."

The Dark Show is being compered by Adam Hills - an Australian comic renowned for his cheery and bouncy stage presence. "I think I'm being used as the cleansing comedy sorbet in between all the filth," says Hills. "I think it's an interesting idea for a show - I once heard someone say that if something is funny, it can't be offensive and if something is not funny, it is offensive. I'm not sure how much I agree, but there are plenty of issues there. As a travelling comic, I hear a lot of material that is very, very edgy.

"Paedophilia seems to be a big one with comics these days and you have to be very careful how you tread. I once heard a comic do a 15-minute routine about paedophilia that was brilliant because it dissected what it was all about and presented it in its rawest state. For some people though, it was just too much and there were walkouts, but I thought the guy was making a valid point. It's doing that - and keeping it funny while dealing with such a horrendous subject matter - that is the real challenge.

"I don't think any subject matter is taboo, but I do think what the comedian's intention is with that subject matter is absolutely vital. Greg Fleet does material about homophobia and race which some may find very strong - but it's the point he's making about those subjects which shines through."

Hills points out how an act such as Jimmy Carr can say something which is indisputably offensive and abusive, but with a smile on his face and with a killer punchline. Similarly, Ricky Gervais can be offensive to ethnic minorities, but with the intent of showing up his own crass stupidity.

"I think context is almost everything," says Hills. "There are remarks I would make to my friends which I wouldn't dare use while doing a show. I can get away with talking like that because they know I'm not a monster and I don't really mean what I'm saying - I'm just making a flippant remark at someone else's expense, which I hope they find funny."

You see this at play in the film The Aristocrats, which is an examination of a time-honoured, ultra-vulgar joke which is passed around among actors and comics.

The set-up and punch-line remain standard; a family entertainment act are auditioning in front of a showbiz agent. When asked to describe their act, they come up with the most transgressive, profanity-ridden explanation possible. There were few complaints about the film simply because the very premise of the work was to try to be as offensive as possible.

As a well-known Australian TV host, Hills was recently asked to present a posthumous award to Steve Irwin - the late wildlife TV personality. "Irwin's wife was in the audience but that didn't stop me making fun of his macho Australian image - which I used to do in my set even before he had died," he says.

"What I noticed was that was everyone at her table was just glaring at me, but I kept looking at her and she found it all very funny. The next day, the press reported that she had walked out because of my 'offensive material', but I got a phone call from her saying how much she enjoyed what I had said about him. People do like to take offence on other people's behalf, and that, in itself, is funny."

yyy Adam Hills hosts Greg Fleet, Andrew Lawrence and Carol Tobin in The Dark Show on Sunday, June 1st, at The Zoo

Bar, Kilkenny as part of Smithwick's Cat Laughs. Full festival details at www.thecatlaughs.com