What comes to mind when you think of folk music? Chances are you'll be picturing, in the words of Charles Hendy of the Mary Wallopers, "Musicians going around wearing fucking tweed!" Equally, chances are relatively high that you won't be picturing hundreds of pints of beer being flung into the air at Brixton Academy to the sound of Charles, his brother Andrew, and the rest of the band shouting so many swearwords at the rich that their wealthy ears are probably burning all the way from Belgravia.

But then, if you listen to the Mary Wallopers and other bands riding the Celtic revival – from Scotland's Brògeal to Ireland's Lankum, you'll realise that folk music (or trad, as it's known in Ireland) has always been about upsetting expectations, getting political, and updating songs for modern times. It's an iconoclastic approach which seems to have resonated with crowds, turning the softly-whispered, knitwear-clad image of traditional balladry on its head and opening it up to a whole new audience.

Helping fuel the wave and riding alongside it are bands like Fontaines D.C., who have proudly used the Irish language and Irish instruments in their songs, and Irish-language rap group Kneecap, who, in the words of Charles, are "making [the Irish language] very sexy.

"Irish culture has always been popular," he continues, backstage at Brixton and preparing for their second show in a row, booked after the first sold out. "But right now, all the musicians know each other, and we all have very similar political views; it feels more unified. Ireland itself is closer to the reunification of a 32-county Republic than it's probably ever been – I think we'll see it in my lifetime."

Reunification or not, it's an identity which is proudly resurgent, and folk hasn't been so contemporary since The Pogues debuted their take on it over forty years ago. "The Pogues said that we're continuing on the tradition, so it's official now," says Charles with a grin. "The trouble with folk," he continues, "is that people started putting it on John Lewis adverts. Folk music isn't about that shite; it's about drinking, fucking, and shooting police."

"Some of these musicians take themselves so seriously," adds Andrew with a groan. "They walk on stage as if they're clergy, with no shoes on or something. Everyone's standing worshipping them, but it's not folk music when the stage is so much higher than people's heads.

"Folk musicians used to get killed for singing their songs, and now people are saying, 'Let's keep the politics out of music'. It's a political fucking tool! It should never be safe, or feel safe, or sound safe."

"The funniest thing," adds Charles, "is when we were touring America, and early on when we toured England, promoters would put on these exact type of folk bands before us. Fuck, I felt sorry for them! They're lovely people and all, but they'd go on stage and be fucking eaten alive because our fans are throwing pints all over the place and shouting at them."

Anyone who has been to a Mary Wallopers gig will know precisely what they're referring to, but their word-of-mouth nature and existing views of Irish trad music mean it's been easy to miss them, no matter how meteoric their rise. They may be near enough selling out two nights on the bounce at Brixton, but their determination to forge their own path means it can be hard to slot them in with other bands, even those who – like them – are embracing their Celtic roots. In part, this is down to sheer bloody-mindedness, but it also comes from their origins in Dundalk, a border town in Southern Ireland which is about as culturally distant from central Dublin as it is from misty-eyed images of rolling hills and turf.

"It was nicknamed El Paso by the BBC," says Charles by way of explanation (after the notorious town on the US / Mexican border). "I don't think it was meant to be, but it's used as a badge of honour by people that live there. It's a factory town; Harp lager was brewed there, Carol Cigarettes were made there, and there's a richness and a rawness in Dundalk that can't help but influence you."

"People have a healthy cynicism there, which is good," adds Andrew. "You can't get too many notions about yourself. There's a disrespect for authority, too – everybody knows someone who does a bit of smuggling or dodgy dealing. Everyone likes a bit of dodgy craic in Dundalk, which suits us down to the ground," he says with a laugh.

Despite this industrial history and the unique culture it, and the town's border location, have left behind, Dundalk's economy has taken a beating over the past few years, with the tech firms who have flocked to Ireland only widening the disparity between the country's richest and poorest citizens. The band's well-deserved anger at the situation finds its way into their songs, with the lyrics of 'The Idler' imploring people to focus on the real villains and to "blame the idler who forgets their history / And says "Ireland is full, I'll look after our own".

"We had a recent election in Ireland," explains Andrew. "And a huge amount of people our age didn't vote, and all the right-wing cunts said it was because we didn't care, but it's because everyone my age or younger have been driven out of the country because they can't afford to live in Ireland. They're in Australia or Canada because of the policies of those people. It's disgraceful that because of the rent crisis and austerity and everything else that Fianna Fáil and Fine Gael [Ireland's two dominant political parties] have created, we can't even change it because we're all being forced to emigrate."

"But it really is our culture," says Charles. "It's in our culture to be an immigrant. A lot of the traditional songs are about dreaming about being in Ireland because they were written by immigrants. That's why it's so frustrating, the thought of Irish people turning around and falling for the same bullshit about refugees that we were subjected to in England for so long. Irish people were forced to sign anti-terrorism waivers to get into the country, and the amount of innocent Irish people who were put in jail or questioned or beaten up by the cops for being in the IRA when they never were – how can people forget that so quickly?"

"It's very much a new thing," adds Andrew. "Right-wing groups from America and England are funding right-wing people in Ireland that are starting to grift and spread disinformation. And it's so sad to hear Irish people talk about immigrants in the way the Irish people were talked about for so long."

It's heavy stuff, but it's a far cry from the drunken revelry of a Mary Wallopers gig, a contrast which is completely intentional. The band play a set consisting primarily of reworked covers of traditional songs rather than originals, and aside from the Palestinian flag draped over part of the stage and some between-song banter, contemporary issues aren't pushed to the fore. Far from sidestepping an issue so big it can feel paralysing, though, this is just a different way of approaching the subject on the band's part.

"Our priority is communicating with people," says Andrew. "There are so many prejudices about being left-wing or having left-wing ideas, people switch off. But when you boil it down, the message we're really spreading is just 'don't be a cunt'."

"It's as simple as that." agrees Charles. "We try to use our platform to raise voices where we can, but really our biggest tool is getting these ideas across through quite nostalgic songs, because they're ideas which have always been there.

"Our fans are normal working people. We would never expect people to want to take part in some Marxist fucking reading group on stage because people are tired and they don't have the time. A lot of that left-wing stuff can get very wordy. Not to say people are stupid, but it becomes very theoretical - I've never fucking read any of it, but I still know what I'm into."

"Our job is to just let you let loose," says Andrew. "People need a break. Everyone knows how bad everything is, but I'd like to think of our gigs as a bit of a community, a bit of respite. It all comes back to accessibility, too. Right-wing people get elected by going, 'We hate paedophiles!' – well, no one likes paedophiles!"

Whatever the topic, both Charles and Andrew eventually bring things back to the live experience. It's not surprising for a band who cut their teeth doing pub sessions in return for free beer, but two albums in, it's an attitude which has started to shift ever-so-slowly. Debut 'The Mary Wallopers' and follow-up 'Irish Rock N Roll' were released within a year of each other, and both consisted mainly of traditional songs reworked by the band. While they more than hold up as albums in their own right, you can tell they were a reflection of the band's gigs rather than the other way around. Album three hasn't come so fast, and when it does, it's likely to be a very different beast. The changing direction of the band was accelerated by founding member Sean McKenna amicably leaving the group in March, with the remaining members keen to try something new.

"The first two albums are a snippet of what we do live, but this one will be more experimental," says Andrew. "We want to spend some time on it and mess around with it, rather than, like, take two weeks and just fire it out.

"Sean leaving is good timing, in a way, because we're right at the start of that third album. Even when you're playing a session or something, every extra musician that comes or leaves slightly changes the way that you approach your playing, so it's an exciting time."

And if it doesn't work out? Well, they aren't exactly worried.

"We started this band when we couldn't afford drink or food," says Charles. "So the fact we've made a living at this and never compromised – even when we were really struggling, we only played one or two weddings, max. Don't do weddings, don't do fucking requests. We had a major label try and sign us a couple of years ago, and we really could have used the money, but fuck 'em!"