Their punked-up versions of traditional Irish songs have wowed audiences. Ahead of recording a third album, the Dundalk band talk about their journey from playing for beer and whisky to this year’s Glastonbury

On a Monday night in the backstreets of Camden, the three founding members of the Mary Wallopers are mere days away from the biggest show of their careers, playing to many thousands on Glastonbury’s revered Park Stage – and to considerably more watching on TV. And how are they warming up for this massive breakthrough gig? By headlining a fundraising evening for the London Irish Centre, to a crowd of barely 100, alongside the Irish Pensioners Choir. At one point, a woman in her 80s called Breda is brought on stage beside them to sing the ballad Slievenamon and they watch like polite grandchildren, humbled by the performance of a singer three times their age. Later, they will be summoned back to draw the raffle winners from a tombola. It’s not rock star glamour in its conventional form.

Four days later, they take the stage at Glastonbury in their enlarged live form as a seven-piece, the bass amp draped with a Palestinian flag, and the crowd is packed, even at 3.15pm. Bleach-haired Charles Hendy sets the tone with a grin: “We are the Mary Wallopers, and this is Bould O’Donahue, a song about riding the Queen’s daughter.” (As a marker of the song’s longevity, the monarch in the version popularised by the Aran sweater-wearing Clancy Brothers was Queen Victoria.) At another point they play a 400-year-old song, The Rich Man and the Poor Man, with lyrics tweaked to incorporate the c-word, to gleeful cheers. They induce moshpits with their punked-up versions of jigs Frost Is All Over and The Blarney Stone, and the crowd seems to be singing along to every word of Hamish Imlach’s witty tale of boozy regret, Cod Liver Oil and the Orange Juice.