The XX have never been quite as glum as popularly portrayed. The first time I met the band’s frontman, Oliver Sim, he’d just made his way back to the Olympia Theatre in Dublin after becoming lost Christmas shopping. He was neither waifishly shy or self-parodyingly gloomy.

“My phone died and I couldn’t figure out how to get back to the venue,” he explained. “It looks like I’ll have to get all my presents on the internet this year.”

He said this with a smile that belied the head-to-toe black in which he was enshrouded. Indeed, despite resembling a Tim Burton doodle brought to life, his manner was chatty – almost effusive. When the Londoners put out an up-tempo third album, I See You, last year the shift in tone made perfect sense to me: it was a truthful reflection of the artists I’d met in person.

“We’re normal people,” Sim had said. “I mean, I know we don’t do ourselves any favours by dressing in black. But no, we don’t go around feeling sorry for ourselves all the time. We like to go clubbing. We do the same things as everybody else.”

“When I was a teenager, my life was one long descent into black really,” he told me on another occasion. “I think black is quite chic. That’s why I wear it. Not because my soul is dark or anything like that. We’re not trying to make a statement and it is definitely not a uniform. If I turn up at the next rehearsal in an orange tracksuit, no one is going to send me home.”

“We were young and it was painful for us to be up there,” is how Romy Madley Croft described the group’s initial incarnation as wincing introverts – a phase they have assuredly moved beyond as they look forward to headlining Electric Picnic.

“It must have been painful to watch too, I imagine. We have received so much love and it has helped us gain in confidence. Nowadays I would like to think we are at ease on stage.”

She blamed a lack of self-belief exacerbated by extreme youthfulness. When the xx began in 2005 Sim, Madley Croft and in-house producer Jamie Smith were barely out of school and found the hype overwhelming. Only after winning the 2010 Mercury Music Prize for their self-titled debut – an extraordinary blending of indie angst and epic house – did they grow into themselves.

“While the person who was too shy to go on stage is gone, I still feel like I know her,” said Madley Croft, sounding wistful about a younger self vanished forever. “We’ve toured relentlessly and shed our ‘childlike’ skin, I suppose. However we still remember what those days were like.”

Electric Picnic 2017 marks their second time closing out the main stage at Stradbally.

“If you’d told us three or four years ago that one day we’d be standing in front of all those people, we’d have run a million miles,” said Sim of the festival in 2012. “There has been a lot of growing up.”

Behind their cerebral dance pop, the trio have always kept a low profile. This has continued into their third LP, for which they have done the minimum of promotion.

“We are private people,” Madley Croft said to me. “We don’t have our faces on any of the band’s artwork. First and foremost we want it to be about the music. We understand that we are recognised now as musicians and that people want to know about our lives. For us, it doesn’t make a difference. The feeling remains the same: let’s put the music first. We’re not craving to be famous. We are private and eager for it to stay that way.”

“I don’t think it’s good for people to know too much about you,” added Sim. “With my favourite bands, I don’t want to have the inside track on every single aspect of their personal lives. We want the focus to be on our music rather than us. Honestly, I don’t think we’re all that interesting as people. We don’t like to overshare, which can be unusual in this industry. In the real world, however, I think it is absolutely normal – it’s the way most people are.”

Sim and Madley Croft have been friends since childhood, having first met at play school. Smith came into the equation later – but shares a deep camaraderie.

“We have rows – but it’s the way I row with my older sister,” he said. “You say what you feel and it’s out there and you move on. The worst situation would be if you held something back and it sort of builds up. We are very direct, especially in the studio. And then we forget about it.

“You see bands putting ads in the paper: ‘drummer wanted’. I could never be in a set-up like that. It would just be so weird. For me, that would be tremendously uncomfortable. I’d hate to be a solo artist too. I remember reading an interview with Adele where she said that touring was the loneliest thing in the world. All of her band are hired, so, really it’s just her. I can’t imagine what that must be like.”