Eight years, four albums, numerous tours and over three and a half million album sales later, the members of The Priests remain as calm as they surely were when they first entered the seminary in Belfast over 30 years ago.

Without wishing to stereotype, Fr David Delargy (52), Fr Martin O'Hagan (52) and his brother Fr Eugene O'Hagan (56) match the identikit picture. The three men are the epitome of what you imagine priests of a certain age and generation look, sound and act: relentlessly sincere, extremely civil, wilfully asexual, exuding a calmness that is more comforting than unsettling.

Their movements are measured, their body language circumspect. When talking about topics that don’t run parallel with their sideline music life, they never raise their voices above a specific level. You sense that passion would run amok on the sidelines of an intercounty GAA match, but nowhere else (unless, perhaps, when talking about same-sex marriage, more of which later).

The three middle-aged Northern Irish men have known each other from the 1970s, when they were students at St MacNissi’s College (now St Killian’s College), Carnlough, Co Antrim. After vocational training in Belfast, they studied at the Irish College in Rome – Fr David specialising in Sacramental Theology and Education, Fr Martin in Moral Theology, and Fr Eugene in Canon Law.

They have been based in Northern Ireland from the 1990s, and each has since been working full-time in their respective parishes (Fr David in St Michael’s, West Belfast; Fr Martin in Newtownards and Comber; Fr Eugene in Ballyclare and Ballygowan – he is also chancellor of the Diocese of Down and Connor). Despite their commitment to their parishioners, however, their lives are noticeably different from other parish priests. These priests have tour managers, they do soundchecks, they sign autographs.

Signed by Sony BMG in 2008, the trio has released three studio albums, The Priests (2008), Harmony (2009) and Noel (2010), and a Best Of collection, Then Sings my Soul (2012). The 2008 release set a Guinness world record for Fastest-Selling Classical Debut Album, surpassing debuts from the likes of Katherine Jenkins and Luciano Pavarotti. BRIT Awards nominations followed, as well as tours of Europe, the US and Australia. At first, they were viewed with the same kind of bemusement you'd afford a novelty act.

"When we released the first album in 2008," says Fr David, the most talkative of the three men, "Ireland was very familiar with the Father Ted scenario, the My Lovely Horse song, and all of that. So people initially saw us along those lines. Seven years later I'd like to think people have realised it isn't like that."

Did they have any qualms engaging with the music industry, a global machine that throws people in the dumpster after they have outgrown their use? Contracts, they say, had to be finely outlined by a music entertainment lawyer. Their primary concern, says Fr Eugene, was that “as priests we have commitments in our parishes.”

“People who had some experience with the industry were saying that we needed to be careful, almost as if the music industry was somehow the dark side, and how were we going to survive by interacting in such an environment,” says Fr David. “But we were very careful to make sure that who we are and what we stand for would be respected. We were very careful about negotiating all of those things right from the start.”

I ask how they balance parish life with touring – never the twain shall meet, surely?

“The music industry wants you to be available all the time,” says Fr David. “We couldn’t do that, as our priority is working in our parishes, so what we try to do is to carve out, quite far in advance, about two weeks of our year to go on tour. It is difficult, as we each have different roles and different diaries, so it takes careful planning.”

Pop stars are prone to temper tantrums and mood swings, so we talk about how a priest’s virtually constant display of civility and decorum can sometimes be viewed as unnerving. Three heads nod empathetically. Fr David admits that being at the beck and call of parishioners day after day can be “very exhausting, very draining – emotionally, spiritually, every way”. Fr Eugene says, “There are times when you would love to say things that come into your head, but it isn’t good if you do that. There’s a certain standard, a higher standard to attain.”

We talk about the occasional loneliness of priests, particularly those who administer and live in rural parishes. Fr Eugene says, “loneliness is part of everybody’s life”, which isn’t delivered in as offhand a way as it might read.

Fr David readily acknowledges that a number of priests live “physically isolated” lives. “It’s easier, perhaps, for priests who live in urban areas to have more opportunities for contact, although cities can also be quite lonely places.”

I ask how they feel about the Republic’s yes vote for Equality. There is a noticeable shift in demeanour, a hum of disquiet and (so it would seem) a confession box full of diplomacy.

“It wasn’t a surprise,” says Fr Eugene, “but at the same time I think the church would have preferred a different outcome. However, the Catholic Church is not the world. Personally, I struggle with the idea that marriage can be redefined. I’m happier with the idea of civil partnership, which recognises the unity of the relationship, its permanence and stability, and so on.”

Fr Martin says nothing. Fr David takes up the gauntlet, and lets rip in a calm if seemingly unforgiving way.

“I’m very much formed by the Biblical tradition of God having created male and female. The openness to new life is a central pillar of what we and the church, and civilisations throughout history – be they Christian or not – have always understood marriage to be.

“And yet here we now find ourselves, at the beginning of the 21st century, where it must be said a relatively small minority of people are very actively and vociferously challenging that, and claiming it on the basis of rights. I’m not sure this is the correct context in which these things should be debated. Marriage is what it is. You can call something else marriage but just because you call it so doesn’t make it so.”

“I think for too long,” says Fr Eugene, moderating his friend’s comments in true chancellor mode, “the church has demonised people who are different, and that’s wrong. It’s a challenge to change perceptions and passing a law won’t change them. I know priests who are gay . . .”

“It can be living hell for people,” says Fr Martin on the same topic, “and we have to disentangle the knots of the past in the hope that there are many who are part of the church and who want to be part of the church, but who struggle.”

The actuality of gay priests, says Fr David, "is still something of a taboo subject, less so now, perhaps, than it has been in the past." He references Pope Francis: "When he was asked about priests who are gay, he responded by saying if a person who is gay is earnestly seeking God, then who am I to judge?"

And with that, The Priests say farewell – three men with a mission, a collective music voice that a few million people cherish, and traditional views, some of which are non-inclusive. The season of goodwill to all men? Theoretically, but just because you call it so doesn’t make it so.

The Priests are at Mount Errigal Hotel, Letterkenny (today), Opera House, Cork (Monday), National Opera House, Wexford (Tuesday), Waterfront, Belfast (Sunday, 20th) and Millennium Forum, Derry (Monday, 21st).