“All roads in Dublin lead to Copper’s,” JC Stewart says, pointing down at the infamous nightclub from his perch on Sophie’s Rooftop Terrace. “And in my experience, that’s definitely never been a good thing.”

‘Sad boys’ – a distinctly Generation Z phenomenon – revel in a nihilistic aesthetic, but the Derry-born singer-songwriter in front of me, who proudly bears his ‘Professional Sad Boy’ nickname, is in surprisingly good form. In fact, like his pal Lewis Capaldi, the gaping contrast between JC’s melancholic, soul-baring sound and his light-hearted personality is all part of the appeal.

“I’d known Lewis for a couple of years,” JC recalls, “and then one night I was coming back from a session when he rang me up. He asked me if I’d go down to the studio, and write a song with him, but I wasn’t that bothered at first. In the end he convinced me, and I brought a few of my mates down. We wrote ‘Hollywood’ from 12am to 4am.

“Nine months later it’s on his album, Divinely Uninspired To A Hellish Extent. I just saw its sold over a million copies worldwide, which is nuts. He deserves it – he’s always been such a funny guy.”

JC is seriously nonchalant for a 22-year-old who spends his days jetting between London and Los Angeles, rubbing shoulders with some of the biggest names in the industry, earning praise from the likes of Niall Horan and clocking up Spotify streams in their millions. Clearly, apart from a brief stint studying International Relations at the University of Sussex, he’s been preparing for this moment his entire life.

“I played in pubs for years, and then I started writing songs for other people,” JC tells me. “There was never really a ‘big break’ moment. It’s all moved slowly but surely. Meeting people like Kodaline and Snow Patrol has helped me massively Your peers are your biggest support network in Ireland, rather than the big companies."

When he was 17, he also found himself working alongside Nile Rodgers and Rudimental on a remix of ‘Le Freak’.

“I just applied for it online,” he laughs. “It was labelled as work experience! I was at school, and I got this email saying, ‘You’ve got to come to London and work in this studio’. It was insane, and I met so many people doing it. Nile’s a good lad.”

More recently, JC joined forces with Grammy-winning producer Fred Ball (Beyoncé, Jay-Z) and songwriter Justin Parker (Lana Del Rey, Rihanna) for his single ‘Have You Had Enough Wine’, released earlier this summer.

“The song is about this really small conversation I had,” he explains. “It wasn’t a shouty break-up or anything, but it was painful. The idea came to me fully formed as a song.

“I’ve talked about it with the girl, and she loved being in the song,” he laughs. “She thinks it’s hilarious, actually. I told her, ‘It’s kind of about you, but not completely’. She was like, ‘Oh, I’m telling everybody it’s about me!’ Alright, do your thing!”

While he’s certainly poised for ocean-crossing megastardom, does he ever find himself getting homesick for the Maiden City?

“I like going back and visiting the northern coast, but I don’t miss it, really,” he admits. “It’s strange times in Northern Ireland at the moment. The DUP can go and do one, as far as I’m concerned. There’s so much amazing music coming out of Northern Ireland right now though, with ROE, Kitt Philippa and SOAK. It really just needs to be pushed into the spotlight some more.”

• JC Stewart headlines The Islington in London (September 23-26). His latest single ‘Bones’ is out now.