"I said, 'Am I speaking too fast, is that it?'" Tiernan remembers. "He said, 'No, you're not allowed to curse. This is one of the best comedy clubs in America and we don't think you should have to curse to be funny.'" Tiernan could see his point, but had never considered that the expletives which are liberally sprinkled through his narrative were there as comic tools. "I try not to think about what I do on stage, and once I'm up there I let it all fall out. If that's language, then that's what it is. It must be an Irish thing. You only notice it when you go abroad, but we do curse a lot. We spit a lot too."

The vignette is a small example of the cultural differences Tiernan is encountering in his efforts to . . . and he finds this term overly dramatic . . . break America.

Yet there is no escaping the unique challenge he has embarked on. One of a slew of Irish stand-up comedians to make a name for themselves over the last decade, Tiernan is the first to go west in search of laughing Americans.

It's a big world over there, five times the size of Britain, where Irish comedians have done so well of late. That's a lot of road, demanding a hard slog and requiring serious perseverance. It's also a cultural mishmash, with values, bogeymen, morals and politics changing dramatically, sometimes just by crossing state lines.

That alone would present a serious challenge for anybody hoping to tap into a national vibe. Operating in a line of work that relies on people laughing, as often as not at themselves and their own mores, it could be daunting. All Tiernan is short of on this pioneering expedition is a wagon, a 10-gallon hat and a pair of oxen.

Even among British stand-up comedians, the USA has often proved to be a gig too far. The only notable success of late has been Eddie Izzard. He threw himself into it by financing four months of loss-making gigs in New York through reinvesting a packet he made on a long run in London's west end. The effort was worth it. From there, things took off at a slow pace, but now he's arriving at the summit.

Whether Tiernan can trot in his wake is something that will become apparent over the next 12 months. He has committed his big adventure to film. The first programme goes out on 8 May and follows him around his initial gigs in New York. In one clip, he is backstage enquiring whether Americans had heard of Lourdes, or Michelle Smith. A blank face suffices as a reply. Religion, as might be expected, is a touchy subject, particularly when clerical sex abuse is introduced.

"We can joke about child abuse in Ireland, people laugh, and sometimes they laugh and cheer at the same time. But over there, they don't like to hear it. One show in New York was in front of an all-black crowd. They were fine with stuff about sex, and the family. But once I mentioned religion they just switched off, as if I was a racist."

That particular gig was played in front of a grand total of 13 people. Tiernan was an anonymous appendage on the bill, a name that meant nothing to the audience. A few days previously, he performed his 100th gig in Vicar Street in front of a packed house, the hordes gagging for Tommy. From hero to zero across the Atlantic. In a business where confidence is all, you wouldn't want to be of a fragile disposition.

"It can be thrilling, " Tiernan says. "One night you're in front of a thousand people, your energy is up, they have come to see you and you're driving it. The next night you're one of 10 comics in a small basement in New York, in front of 13 people who don't care if you're there. You're speaking to them in this weird accent, but my thing is the same. I have to make these f**kers laugh. Are my ideas funny to people who never heard of me? Can I get a black guy or a Mexican or a couple of white people from Manhattan laughing?"

Whatever happens, his confidence is unlikely to take a hammering. A while back, his parents showed him an old school report they'd stumbled across from 1977, when he was nine. "Tommy has great confidence even though he's not entirely sure what he's talking about all the time, " the teacher wrote.

"That is how I am, how I'm built. That is the shape of me, " he says.

A Navan man by birth, he moved to Galway soon after finishing school. He drifted into comedy, playing local gigs until it became apparent he had the makings of a talent. He won a Channel 4 comedy award in 1996 and the coveted Perrier two years later. Combined with an appearance on Father Ted, the awards raised his profile in Britain and off he went, moving steadily to the top of the class.

Along the way there were a couple of appearances on the Late, Late Show, deemed "controversial" for their irreverent approach to religion. They shouldn't have amounted to a hill of beans.

The last appearance in October 2004 prompted uproar, yet the facts show that RTE received eight complaints from a viewing audience of hundreds of thousands.

He is adamant that he didn't set out to offend or whip up controversy.

"All I did was perform stuff that was going down well in the stand-up routine around the country, " he says. Last year he performed probably the biggest stand-up gig undertaken in this country, appearing before 5,000 people in the Marquee in Cork, as part of the City of Culture festival.

This year he will spend four-and-a-half months on the road in the states. He has already gigged on both coasts, and inland in the likes of Nebraska and Pittsburgh.

Comedy has the ability to penetrate pretensions, and Tiernan's experience is revealing.

"Liberal states are worse in terms of accepting ideas because they pride themselves on ideas and are more politically correct. In blue-collar states you've guys who work hard and go drinking and want a laugh.

"In California I did a thing about the Indians and what it would be like if they had become the colonising power. In Pittsburgh and Nebraska, they thought it was funny. In California, it was like, 'You can't call these people Indians, they are the brothers of mother earth.'

"Then white Californians are alright about religion, but they don't like the paedophile priest stuff. With blacks, you can't mention religion at all. But the adventure is getting them to a place that they're not comfortable with but where they'll still laugh."

The adventure might be awfully big, but it's not all beer and skittles. He is a father of three children, and he shares custody with their mother. The road doesn't make allowances for parental leave. As a result, he must balance his commitments. Singleminded pursuit, something which usually distinguishes serious high achievers, is not his station. He wants to do well in America, but only if the price is right.

"I'm always asking my kids did they miss me when I come back from a tour. Last time it was three weeks and they said they kind of missed me, but not too bad. I've six weeks in the summer and they say that's a bit long.

"It's not something I've resolved or feel comfortable with but I don't think it's as bad as I feel about it sometimes. I miss them hugely and I think I'm projecting onto them that they miss me by the same amount.

"In that way I'm not single-minded and that might go against me in the work area of my life, but not in my life as a whole."

If the road can impact on family, it can hardly be great for emotional health either.

Apart from the hotel life and unmoored times, there is the additional elements that affect performing artists. The highs and lows, the silence that follows the applause, the day's rhythm dictated by performance and reaction. Wherefore the tears of a clown?

"Sure I get the blues when I'm performing but you just have to learn how to deal with it. There is the euphoria of the stage and then a crash. It's probably inevitable, but it's not something that comics usually talk about.

"But look, the natural highs and lows of being a stand-up aren't that huge. It's not a self-indulgent, tortured life like that of a Russian novelist. I would be quite a mellow person anyway. Maybe I'm a bit down after performing, but it's not that dramatic."

He has a similar take on the insecurity issue that must drive anyone requiring affirmation from applause.

"I don't question it and it's not something I'm going to solve, " he says.

By the summer, he hopes to have made some inroads in the States. He is booked for a slot on David Letterman's show. There was a time when that signalled arrival.

These days, he is told, you have to appear on Letterman, Leno, Conan O'Brien, and go back again and again, before even glimpsing the plateau of national success.

No matter, he'll drive on. Is he, to pose an appropriately American question, a happy person?

"That psychotic need to be happy is why Americans are all insane, " he says. "They're very positive during the day but the TV shows at night are all about murder, rape and crime. Hollywood is a death factory. It would be amazing if there was a week of death-free TV in America. It would be great not to feel the pressure to appear happy."

And indeed the term 'happy camper' is not one that you would apply to Tiernan. He is lively, funny, bright and pleasant company, but somehow leaves the impression that contentment is a harbour he has yet to anchor at.

Behind the jokerman, there is a serious dude at work.