"There's footage of him in his early twenties walking around the place fine. It's just a silly little sketch, but the American audiences I was playing to would not respond at all."
Tiernan is puzzled by that firm line in the sand. But he's equally puzzled by the scale of America, by the stories that come out of the place, and by the success of people like Christian self-help guru Joel Osteen for example, an author of books with titles like Become a Better You.
"I talk about Joel Osteen in my act," Tiernan says. "In America this self-help preacher is considered a superstar. But because of the way he talks in Ireland we'd say he was retarded. Odd that there's such a gulf in perception between us, isn't it?"
It was the booker on the David Letterman show who set him straight. "You can't say retarded here," the booker told him, "because you can't make fun of someone who isn't there to defend himself."
Tiernan was impressed by this forthrightness. "There's a fundamental decency in America that is probably a great thing to live by, but in terms of comedy it's deadly. Comedy needs to be reckless," he adds.
Although Tiernan admits that he gave up drink four and a half years ago and lives with his wife and three children in Galway, he hasn't mellowed at all. Asked what he does for recreation on a Friday night in a city that is basically wall-to-wall pubs, Tiernan replies that he's happy to be a dad.
"In real life I'm a much calmer person, I read immensely heavy books, I listen to Leonard Cohen and Bob Dylan, and I'm a dad. I like the contrast. It's a simple enough idea. On stage you're a wrecking ball - in real life you might be a gardener."