With Under the Influence wrapped, recorded (and on the shelves just in time for Christmas), and the Paddy Crazyhorse tour well underway, one might think Tommy Tiernan has his routines firmly nailed to his bedroom wall alongside his extensive hat collection. But instead, “every performance is like a rehearsal,” according to Tiernan and the ability to improvise is just as important as the months of preparation that go beforehand.

“Even when you decide to stop touring with a show, it’s still in a state of flux” he explains. “There were stories in that show [Under the Influence recorded at The Water Rats, London] I don’t think I was doing for more than a week or two. Then there’s other stories I was doing for a year and a half, you’re always changing it”.

Another aspect of live performances that are guaranteed to change every time are audience reactions, and if watching him perform in Under the Influence is anything to go by – with a sly look around the audience after a joke and brief pause before his genuine delight at their laughter - this is something Tiernan is keenly attuned to. “You have to care, otherwise there’s no point of being on stage,” he says. “It has to be vulnerable. And you make mistakes because you’re vulnerable, but you also make fantastic discoveries. That’s part of the attraction. You want to be slightly unhinged, slightly unprepared, and out of your comfort zone.”

“It’s amazing, you could be in front of 2,000 people and there’s one woman not laughing and she just becomes the big bee in the room. That’s the only thing you can see or relate to. That’s why often towards the end of the show you see a comic just turn to someone and say, ‘what’s wrong with you?’.

Like most comedy, Tiernan’s stand-up is based on hot issues and outrageous standpoints but in today’s climate, are there any topics that are off-limits?

“I think if you approach them comedically, you can say anything,” Tiernan reflects.” I don’t think there are any subjects you can’t talk about. There are approaches that are unpleasant to listen to, you can talk about something in a very derogatory and hurtful manner and I think the audience would let you know very quickly.  I went to see Bill Burr in Dublin a couple of months ago - someone who is not careful on stage - but there’s something about his approach that is just delightful. You want to hear him talk about the dark stuff.”

Speaking of risky material, an inescapable landmine for any comedian is the necessity of bringing their personal life on-stage, and Tiernan’s set sees him go from the marriage bed to the doctor’s office with no shortage of family shout-outs. But unlike one of Tiernan’s favoured comedians Bill Burr - who recently revealed his wife doesn’t enjoy his material - Yvonne Tiernan not only acts as Tommy’s producer but can often be found in the audience. ”I have a comedy wife”, Tiernan explains. “She bears very little resemblance to my actual, human wife”.

Another aspect of his personal life that comes up in Under the Influence is Tiernan’s decision to begin drinking again. But just how ‘under the influence’ is Tommy while out on tour?

“Not at all. I’ve found sometimes even having a coffee at the interval distorts your spirit. So I’d have a thing of trying not to have caffeine after two o’clock in the day, trying to eat four or five hours before the show. It’s a bit like going backstage at the Pogues and it’s all yoghurt and Pilates reformers!”, he laughs. “But it’s whatever people need. When I was young of course, and you think you’ve discovered the key to all creativity, but I can’t imagine performing high or drunk in a way that would be interesting beyond... ten or twelve shows. It’s just about getting yourself in that state beforehand”.

So without liquid courage, what happens when Tiernan finds he’s just not in a funny mood?

“Often the great cure for it is honesty. Walking out in front of 2,000 people and saying ‘I don’t want to be here’ gets the ball rolling. It’s when they shout out ‘we don’t want to be here either’ that you know you’re in trouble.”

Under The Influence is out now on DVD.