‘The older you get,” asks Tommy Tiernan, “do you get lazy, and complacent? Or do you get more joyfully reckless? And how do you stay challenged?” Such are the questions you ask yourself when contemplating a quarter of a century as a standup – or at least, you do if you’re Tiernan, a titan of Irish comedy now enjoying newfound prominence thanks to his role as Da Gerry in the sitcom Derry Girls and his acclaimed RTÉ chatshow. Ahead of his new UK tour, Tiernan couldn’t sound further from complacency, as he chews over every questionI pitch down the phone line, and frets – from the perspective of a 50-year-old maestro – at the process and point of standup.

It is not a hilarious conversation; this father-of-five does not come across as a frivolous man. Nor would you expect him to from his comedy, which hurls itself at the everyday (God, sex and family are signature themes) with a fierce purpose. In recent shows, wild of beard and eye, he’s styled himself more as a philosopher-priest than mere jokes man – now roaring, now whispering, delivering observational comedy about our lives and loves as it might be inscribed at the altar of some druidic temple. The Irish accent helps, with a side order of lyrical Celtic mysticism (“Did you ever listen to a field? At night?”) most notably in 2014 show Stray Sod. Even when he’s rubbing you up the wrong way – he sometimes does – you can’t deny Tiernan is the real electrifying deal.

Mind you, “electrifying” might just be a phase he was going through. You can divide his – or any artist’s – career into phases, he tells me. “I can see phases I went through as a standup, some a lot better than others. You can be stuck in a phase for five or six years. But when you’re in it, it’s the only way you can work.” Now, he feels like he is on the threshold of a new phase altogether. “Something a bit slower,” he says, hesitantly. “Sometimes pace can work as an obstacle to flow. I’ve done standup so often when I’m manic and loud but that can shut down inspiration.”

Facebook Twitter Pinterest Finding the flow ... Tommy Tiernan. Photograph: Johnny Savage/The Guardian

“It’s about finding the flow,” he says. “In this interview, I’ve no problem talking. You ask me a question, I might waffle on for 10 minutes. Afterwards I’ll be going, ‘How can I talk as freely on stage as I do to Brian over the phone?” This thinking may derive from the success of The Tommy Tiernan Show in Ireland, the conceit of which is that Tiernan doesn’t know who his guest is (and therefore, what questions to ask) until they appear on set. The off-the-cuff conversations that ensue (such as “What is the nature of happiness?”, to Mrs Brown’s Boys star Brendan O’Carroll) have won Tiernan considerable praise. “He’s that rare thing,” wrote one critic, “an effortlessly instinctive interviewer, genuinely curious about people, who never fails to draw the best out of his guests, famous or obscure.”

Small wonder he might wish to import those qualities of calmness and responsiveness into his live act. Spontaneity, too – an important quality to Tiernan. “I’ve always been attracted to people such as David Letterman, who can be funny in conversation, and who don’t have to repeat the same thing the next night. That is one of the real tests of a comic, your ability to be funny in the moment.” Tiernan took this interest to the extreme some years back, touring a wholly improvised standup show. But it was too much, he says now, for himself, and for audiences. “In terms of quality, they had a hit rate of 50 or 60%. That means 40% of the time, you’re walking off sensing the audience have been slightly bored or disappointed.”

But finding a way to be spontaneous, or to stake out a space for play, remains a priority for the man from County Meath (where he went to school with fellow comic Dylan Moran – they later won the 1998 and 1996 Perrier comedy awards respectively). For his new show Tomfoolery, Tiernan has devised strategies to keep himself fresh on stage. He’s trying to make standup feel unlike work. He’s studying other practitioners of improvised performance: the dreaded phrase “I’ve been listening to a lot of jazz” is uttered, but so too, is a hymn to Michael McIntyre’s spirit of fun.

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It’s about keeping open the possibility that something surprising might happen in the moment. Yes, Tiernan has written plenty of new material, a process he recently described as “hellish”. But “if the show is entirely decided by me in solitude, I might as well write it down and post it to the audience. Something has to happen when the performer stands in front of the crowd.” That something is hard to pin down – but “the fact that I can’t define it makes going on stage every night interesting to me,” says Tiernan. Nor does he shrink from ascribing sacred qualities to the audience-performer encounter. “I could say: it’s an opportunity for communion, for myself and the audience to open a chakra that doesn’t normally get opened. But then you hear yourself”, he says, and lets the chakras fizzle into silence.

The joy with Derry Girls is to have become a team player. That’s been a real delight for me

“As long as the show stays slightly out of control, then there’s hope for it.” Out-of-control is another quality he prizes highly, hymning “a sense of the outlaw in standup. That you say the first thing that comes into your head. That you’re this unruly presence, who doesn’t play by the rules.” Is that identity harder to sustain in the hypersensitive, censorious present moment? Tiernan has previous with causing offence, notably in 2009 when he embarked on a wilfully provocative rant about the Holocaust as part of an onstage Q&A.

“But I feel we’ve passed through the heat of that now,” he says. “Maybe six or eight years ago it was burning hot, that question of how seriously do we take what a person says on stage? Now there’s an awareness that if it’s on stage, we oughtn’t pay too much attention to it.” I’m surprised he thinks that – although it may be that the catch-all offence-taking of a few years ago has focused more tightly in 2020 (see the criticism of Dave Chappelle and Ricky Gervais) on issues around identity politics. At any rate, says Tiernan, “if you say something the audience finds distasteful, I – and the vast majority of performers I know – am absolutely prepared to confront that in the room. Rather than have it taken out for forensic examination the following day. The following day it’s always indefensible.”

Facebook Twitter Pinterest ‘I still feel up for the adventure’ … Tommy Tiernan

He doesn’t anticipate great controversies to arise from his upcoming tour, while warning that “tension attracts comedy. Whatever the culture is tense about, you do drift there as a comic.” To make people laugh? Or to communicate something meaningful? A bit of both, says Tiernan. “If you view the crowd as people who have demanded a certain thing of you – ‘Just make us laugh’ – you could end up resenting that. And if you think of them as people gathered to hear what you have to say, that can end up being a very unfunny show. If you lean too far in either direction, you’re screwed.”

It’s about getting the balance right – between script and spontaneity, gaiety and profundity. And now, between the first half-century and the second. “If I look at my career so far as a mouthful of sweets,” says Tiernan, “there’s more bitter ones in there than I would have liked. But I know that I’m excited about getting on stage tonight. And that is a nice place to be.” His recent success in Channel 4’s Northern Irish schoolgirl sitcom Derry Girls – his first acting job for 20 years – has certainly offered Tiernan a tasty sweetie to suck on. It helped his last tour become the best-selling yet, and provided a blissful break from the pressures of working solo. “The joy with Derry Girls is to have become a team player. That’s been a real delight for me.”

“But even though I’ve been doing standup for 25 years,” he adds, “I’m still engaged with it, still frightened by it, still excited. It’s an art form that’s constantly developing, but I don’t have the sense that it’s reached its potential. I don’t know,” he tails off, as standup’s secret lingers – to the last – just beyond his reach. “I still feel up for the adventure of it.”

• Tommy Tiernan’s Tomfoolery is at Birmingham Town Hall on 3 March, then touring.