It's a big night at the Carinish village hall, on the little island of North Uist in the Outer Hebrides. A capacity audience of 200 - almost one in seven of the entire population - have turned out for a musical charity show to buy beds for the old people's home. Outside, the curlew and redshank are calling across the island's barren mosaic of moorland and tiny lochs and, on the stage, there's an impressive reminder that this is still a bastion of Scottish Gaelic culture. There are rousing fiddle tunes from schoolchildren, songs from a local teenager, and "the highlight of the concert", an 84-year-old singing powerfully, with backing from his 12-year-old son on accordion. "Surely you mean grandson?" I ask. "No, it's his son."
In the midst of all this, there are songs from "someone you all know": a girl with an exquisitely cool, clear voice, who is in her late 20s, jokes in Gaelic with her audience about the differences between North and South Uist, and is accompanied by her husband, Eamon Doorley, playing the bouzouki. Julie Fowlis is North Uist's biggest celebrity, this year's BBC folk singer of the year and the best-known Scottish Gaelic singer on the planet. Everyone here knows her songs. Many deal with local characters and events, and several have been learned from those in the hall. Sometimes, she says, she gets nervous singing in North Uist. "When I'm away, no one knows what I'm talking about, but here people know what the songs are about, and who they are about. I've learned a few songs from old archive recordings, but most of them I've got from real living people."
A week earlier, a North Uist man had handed her the lyrics of a song written by his father during the second world war, but he didn't know the melody. "So I tried to track down the tune," says Fowlis. "At the concert, my primary schoolteacher, a great singer, Isa MacKillop, said 'It goes like this'. So in the space of six days, I had the words and tune, and the song was revived."
She is, she says, a "song junkie" obsessed by the "pressure to do it right, be true to the tradition and the language and, most importantly, to the people who have given me the songs. It's like giving you a physical thing, giving you a gift, passing these things on - and the most important thing to me is that they are happy with it."
Fowlis learned to appreciate North Uist by leaving it. Her mother was a Gaelic- speaking islander, from a family of fishermen and crofters, while her father was from the mainland but for many years ran a hotel in North Uist. She grew up "listening to both Radio 1 and Gaelic radio, playing the pipes and dancing", but had to leave the island at 13, when her father took a new job in the Highlands. She wanted to study music, and after being told to learn "a proper instrument", the oboe, went on to study musical performance in Strathclyde. "But I didn't enjoy performing with an orchestra. I got really nervous and sick."
She started thinking of the music she grew up with, playing pipes and whistles in bars "to earn a few quid", and spent a year improving her language skills at a college in Skye before becoming the development officer for Feis Ross, an organisation that supports Gaelic music and song across Scotland, partly by organising music projects in schools. She continued to perform, both alone and with friends in the band Dochas, and was by then doing "tons of gigs, touring all over the place and trying to hold down a full-time job". She was awarded a grant by the Scottish Arts Council to research and record an album of songs from North Uist.
She never intended to be a full-time Gaelic singer. "I'm generally quite a sensible person, and the life of a musician didn't feature in my planning at all. I wanted a steady job and a house. But I did something I wouldn't normally do - I just jacked in the job."
Now she has brought Gaelic song to a following that's far wider than the language's 60,000 speakers in Scotland ("less than 1% of the population"), and is popular even among audiences who can't understand a word of her songs. She agrees that the success of world music has opened a new market for those who don't sing in English, "and I think people are tired of the same churned-out music".
On her most recent album, Cuilidh, Fowlis is backed by a classy band that includes Doorley, along with the mandolin star Chris Thile and John McCusker on fiddle. Fowlis sings and plays whistle. All the songs are in Gaelic, most of them from North Uist.
Driving down the island's narrow lanes the next morning, she points out the musical landmarks. Over there, near the coastal bird reserve at Balranald, is the former home of Jessie, a woman whose elopement to Skye features in two songs on the album. And close by, between the lochs, there's a monument to the "Raiders" featured in yet another song, Oran nan Raiders. Now remembered as local heroes, these were islanders who were promised land if they fought in the first world war. Given nothing on their return, they seized the land themselves. "One of their sons taught me the song," Julie says. "He was at the concert last night, too."
While keen to promote Gaelic culture, she's wary of getting too involved in the politics. "Alex Salmond has been good in taking the mantle of Gaelic-ness of late, but in a lot of ways, Gaelic has been supported by Westminster; I know a lot of Gaels who have a resistance to nationalism and the SNP because of that." Has Alex Salmond contacted her? "Yes, but I was not available at the time. He's a brilliant politician but I'm sitting back and waiting to be convinced. Most of the time my thinking is smaller, about Uist or the Hebrides or the Highlands, not about Scotland."
But whether she likes it or not, she is fast becoming an international celebrity. This summer, ahead of her first American tour, she is playing at a batch of UK summer festivals, appearing alongside Pete Doherty, Shane MacGowan and Lou Reed on the Rogues Gallery tour, reworking Blackbird in Gaelic for a Beatles tribute set, and recording an album exploring the links between Scottish and Irish Gaelic songs. Would she ever record a non-Gaelic song? "I could hardly sing you an English song from start to finish, except for something really embarrassing like a Dolly Parton number. The songs I know are from here, and I'm fascinated that I can sing a song from the 12th century that has survived intact."
· Julie Fowlis performs at the Cropredy festival, Oxfordshire on August 9 and at Aldeburgh, Snape Maltings on August 24
