Waiting outside a bar that they're just barely old enough to enter legally, Heathers – south Dublin twins Ellie and Louise MacNamara – certainly don't look like world-touring musicians. Unassuming in hoodies, they decline the offer of a drink, and begin to talk about the impact on their lives of their first album, 'Here, Not There. I ask them to describe in five words how things have changed in the last year. They crack up. It's an impossible task. Almost immediately after they finished their Leaving Cert exams in 2008, they upped sticks for a tour of the US, sharing a couple of shows with Kimya Dawson. When they returned, the critical praise for their debut, written in Louise's bedroom, was growing and growing. They hooked up with Choice Award-winning Super Extra Bonus Party to record one of the stand-out tracks on SEBP's excellent second album, and embarked on a second tour of the US. They played Oxegen. "Em... Busy. Stressful. Different. Amazing. Notwhatwethought," are the five words they come up with.

"We just thought we were writing songs for us and our friends. We never thought we'd play any gigs. We thought we'd play our friend's 18th and that would be it," says Louise, as they gear up for next weekend's Hard Working Class Heroes festival in Dublin. "It just happened," Ellie says, shaking her head. "Crazy." These two are extremely likeable, sort of bashful about their achievements and emitting a kind of wide-eyed earnestness that's extremely rare for musicians, but understandable for a band so young. They talk frankly about the fights they get into, about the spells of writer's block that have marred sessions of writing, about the huge amount of pressure they feel to follow up an album they never thought would go anywhere and ended up being pretty big in Irish music circles. Older observers within the industry (and it's not exactly hard to be older than Heathers, they're just 19) nod sagely when discussing them as "ones to watch" with "huge potential". They both speak in that teenage Dublinese of "and stuff", "like", "I dunno", "emm.... "

Louise paints a picture of what it was like to jump from the exam hall to a US tour aged just 18. "To go from school to touring wasn't weird. Me, anyway, I was really excited about going on tour, but I kind of put it out of my mind until we actually went. We were also kind of nervous. We were touring with a band we really didn't know at all. We'd never been away on our own for that long either." Ellie adds: "I don't think it ever really hit us. It feels a bit surreal."

On their second American tour, they made more friends and things weren't as rushed because they didn't have to endure the vast distances of the west coast. Their day went like this: "Wake up in the morning in someone's house we mightn't even know at all. It could be absolutely horrible. This tour we had some hilarious ones. Have breakfast. Drive for ages. Take toilet breaks. Arrive there [at the venue] late. Finish. Then everyone would want to party with us but we'd be too exhausted." Too young to play clubs and bars due to drinking laws, they played house shows, all-ages spaces, basements and community spaces. Heathers are part of a young and vibrant Dublin DIY scene spearheaded by Dylan Haskins who now runs an all-ages alcohol-free space in Temple Bar called the Exchange. Some of their early gigs took place in Haskins' semi-detached house-cum-venue (Hideaway House) in the suburb of Deansgrange, and their album was released on Hide Away Records, Haskins' label.

Their American appeal is obvious. Their voices blend perfectly (which makes sense, being twins.) Their songs are simple, melodic, catchy, sincere and intense. Right now, they're trying to get it right again on a second record.

"We're writing our new stuff. It has been going ok and not so ok. It's not like when we were writing our first album. When we were writing that, we weren't expecting anything," Ellie explains, saying they had to change the setting of where they write because they kept getting into fights when they'd write in Louise's bedroom again.

"We went down to Clare to a house down there and it was a lot easier [to write]. I think it's just because it was just the two of us. We had no distractions."

Louise goes on: "We're really critical of ourselves. There are lots of bands who will write tons and tons of songs and throw them out. But we're only writing songs that we want to release or put on an album."

They don't come up with songs every day like their friends in other bands. They sound a little worried. Maybe they haven't found exactly the right situation, or setting, or influences that will work for them the second time around. Sometimes when bands talk about the difficulties they're having, it's a cause for concern. But for Heathers, it just seems part of their creative process and a minor bump in what will probably be a long stint in music for both.

They're chipper about challenges. And before they start producing new material, their existing songs still sound sublime. Their DIY and upbeat ethos encapsulates what the HWCH festival is all about; those hidden gems of bands doing it by and for themselves, homegrown talent that is very much worth nurturing.

Top 5 things to do at HWCH

1. Tweet

HWCH bands will be popping up in surprise venues around Dublin for special acoustic sessions. To keep track of who's on and where, monitor www.twitter.com/HWCH09

2. Banter

Irish Times music man Jim Carroll will host a talk shop with the Tribune's own Una Mullally and Ireland's best music blogger Nialler9 at 8.30pm Saturday in Twisted Pepper, Abbey Street. At 'Meet the New Media Cats', Una and Nialler will talk about blogs, Twitter and the established media, before battling it out with two DJ sets.

3. Gig

Check out Villagers at Andrew's Lane Theatre on Friday night. Newly signed to Domino records, Conor O'Brien (ex-The Immediate) and co have had critics in serious praise mode.

4. Learn

HWCH is a good opportunity for bands to pick up industry info. 'Meet Your New Best Friend', a panel of record label heads taking place at 2pm Sunday in the Button Factory, Temple Bar, will feature experts from SXSW, Domino and more.

5. Snap

Photography has always been a key element of HWCH and a giant roster of music photographers will exhibit at Film Base, Temple Bar over the weekend.

99 Irish bands play HWCH 16-18 October. Weekend tickets cost €40 and nightly tickets cost €18.50. www.hwch.net.