Beverley O'Sullivan is young, blonde and glamorous. In other words, she's pretty much what you would expect from a budding pop star. Except for her new hearing aid. "I haven't heard of any other popstars with a hearing aid," O'Sullivan says with a laugh. "I expect I'm quite unusual. But I'm delighted, as my ear's so bad, I have to do something. I'm hoping it should make all the difference. Mind you, it's a bit mad. It's like I'm an old woman."

It may not be mad exactly, but it is a little unusual. Yes, there are other pop stars with hearing difficulties, but these have usually developed after years of exposure to loud music on stage. Most famously, the Who's Pete Townshend has had severe problems since his hard-rocking days. He not only has tinnitus, but is completely deaf in one ear from an explosion when drummer Keith Moon blew up his drum set live on stage. Other hearing-impaired musicians include the late John Entwistle, also of the Who, Fatboy Slim (aka Norman Cook), Ozzy Osbourne and Phil Collins.

By contrast, O'Sullivan, 23, is just starting her career in the new Irish band Fifth Avenue, yet she has already lost 45% of her hearing in one ear and 30% in the other. Her ears hurt so much she is constantly on painkillers, washing her long blonde hair is a major operation (she simply cannot let water get into her ears), and her choice of career has only made things worse.

"I've been on stage since I was three and this is something I have always wanted to do," she says. "If I have to take painkillers, so be it. I'm really determined and I will keep going."

O'Sullivan has unilateral conductive hearing loss. Sounds are not conducted properly to her middle ear and her sophisticated new hearing aid - the world's smallest inner-ear aid, apparently - helps hugely. At £2,500, it should do. It is possible that in the future she may need another aid for her "good" left ear, but whatever the problems now, they all started when she was a child.

"I had really bad earaches," she says with a grimace. "I was only about three or four and I was screaming my head off."

O'Sullivan had a glue ear, a common ailment in young children and one which can cause infections and inflammation of the middle ear. Her parents, who took her to a specialist near their Dublin home, were told that the problem - and an excessive build-up of wax in her right ear - had led to hearing loss through a perforated eardrum. That perforation has got larger and larger over the years, causing O'Sullivan's hearing to deteriorate.

"I was always in pain as a child," says O'Sullivan. "I couldn't hear brilliantly and I always had to sit in the front at school. I missed a lot, but I got used to it. I can't hear great now either. Things are a bit mumbly."

The perforation in O'Sullivan's eardrum also made her more prone to infections. These infections - and the subsequent inflammation - led directly to her later problems, including the collapse of one of the tiny bones of the middle ear. These bones are vital for carrying sound vibrations.

When you talk to O'Sullivan, it is not immediately obvious that she has hearing problems. She says she's fine on a one-to-one basis. Trouble comes when people talk away from her left ear - the good one - or when they are at the other end of a room. "It's an effort," she says. "I always have my mobile phone turned right up. But it's the pain that's the worst thing. It feels like there's something in my head pounding away. It's very, very sore."

In her teens, O'Sullivan's ear problems seemed to ease off. But when she was 17, disaster struck. The terrible pain came back and pus started pouring out of her ear. She was, she says, "really scared". She was informed that she had an abscess in her problematic right ear. It was, unsurprisingly, connected to her earlier ailments, and the fluid had to be drained out. It was an experience so painful that she still looks uncomfortable recalling it six years later.

Although she says her hearing is nothing to be ashamed of (she would, she adds, be happy to be a poster girl for any of the deaf charities), O'Sullivan does admit that she didn't tell the band about it when she auditioned. Instead she struggled on until it was impossible to ignore any longer.

"I was afraid to tell them. I didn't actually say anything until six months later when we supported Westlife at a gig and my ear was really hurting. I was in such pain, I had to explain why and they were really shocked."

The group have been supportive, O'Sullivan says, although they sometimes get frustrated when asked to continually repeat themselves for her. It's also problematic when she is on the coach, as she often cannot hear what's being said if it's not near her. "When I'm singing I can hear myself fine," she says. "But when we're doing a cappella, I need to have the others right next to me. And when we're performing I need the monitors to be up loud. I sound like such a diva, but it's impossible otherwise."

O'Sullivan's ear problems are clearly not helped by performing at loud concerts and flying round the world. Flying, in fact, is a nightmare, as she finds it hard to cope with the pressure changes when in the air. Fifth Avenue have not yet made it in the UK, but they have had numerous hits in Ireland and spent a year travelling around Europe as they attempted to make it big. Each flight, however, was agonising for O'Sullivan, and one was particularly bad.

When the band was told to fly to Spain to shoot the video for their first single, Spanish Eyes, O'Sullivan was told by her specialist she wasn't well enough to go on a plane. She ignored the advice. "It would have meant not being in the video, the first we'd ever done," she says. "I had to go."

She flew and her ear was incredibly painful. Her hearing also felt worse almost instantly. When she returned to Dublin, her specialist told her the short flight had cost her another 10% of her hearing.

"I really regret that now," she says. "But I'm so stubborn. I was desperate to go. We have to fly so much, and I just try to deal with it. I take sleeping pills before flights, even if they're short ones, but I'm also trying to cut down on flying unless I really need to."

Necessity, of course, is relative, but O'Sullivan appears to have learned some sort of lesson. The band are now based in London and she is keen to stay put for a while. Even her trips home to Dublin are limited.

"What I want to do now is break England," she says. "We've done Europe. I want to be successful here and then decide what to do with the rest of my life. This does affect me - my ears throb when I come off stage. But I can't imagine not singing.

"It's great in Ireland because we have all these fans. People come and wait for us at the airports. But here no one has a clue and we're focused on changing that. Once that happens, I'm not sure how much longer I'll keep going. I do know that health is important too."

· Fifth Avenue's single, Spanish Eyes, is released on September 13.