Fiona Mackenzie on the road to Cruinn

"I find it a bit strange hearing the recorded version now," says Fiona Mackenzie, whose extraordinarily passionate keening of the song's melody, a fine example of Gaelic singing's deep connection with bagpipe music, must have played a large part in Manus Mo Rùin's effect on the awards' judges.

"The original idea was to layer vocal parts by all four of us and make it like the Gaelic psalm singing you hear in the Free Church on the islands, and that's the way we do it now. But we ran out of time in the studio when we were making the album and decided to release it as it was, with just me singing."

There's something altogether serendipitous about the song. Mackenzie and her partner, Dublin-born Brian Ã" hEadhra, whose idea it was to put Cruinn together in the first place, were working with musician-composer-producer Jim Sutherland on his mammoth Struileag project in the Castlesound studio in Pencaitland, near Edinburgh last summer when they found themselves with some time off and a spare room to work in.

"We were putting ideas together for what became our second album, Stòras, and we'd been talking about taking something from Carmina Gadelica, the collection of folklore collected by Alexander Carmichael in the latter part of the nineteenth century," says Mackenzie.

"We had the book with us and when we read Manus Mo Rùin we found it so powerful that we thought, that's the one we should set to music. It's a prayer to Magnus, the great Orkney saint who was slain in 1115 AD and it's almost like a memory from a past life.

"So we wanted the music to have that same ancient quality and it came together really quickly, which doesn't happen very often when I'm writing songs, and to get industry recognition for it is a great surprise as well as a real honour."

Mackenzie has a history of musical surprises. One of four sisters from Gress on the Isle of Lewis, she grew up singing in Gaelic choirs and at family get-togethers.

Her father, she says, had a good voice but was more interested in getting a choir organised in the local community, where he had arrived as a business studies teacher.

"Dad's not from Lewis but he loves Gaelic singing, as well as a lot of other music we heard around the house, and being a teacher he had a great way with kids, so he was able to persuade people to get involved in singing," she says.

"We used to do quite well at the Mod and he taught my middle sisters, Gillie and Eilidh, who competed in the Mod as a duo, their harmonies. We had an aunt and uncle who sang Americana with really good harmonies as well and we used to try and copy them, just singing for fun really."

In her mid-teens, after one particularly dispiriting experience at the Mod, Mackenzie decided that she would never sing in public again. Or so she thought.

She moved to Glasgow to study English and Scottish literature and had no notions about singing, let along joining a band, when Orcadian fiddle and guitar dynamos, the Wrigley sisters got in touch, looking for a Gaelic singer for their then-new band, Seelyhoo.

"These days, young traditional musicians can learn stagecraft, how to work in a recording studio, everything they need to be professionals really, at the centre of excellence in Plockton and on the Scottish music course at the Royal Conservatoire of Scotland but I'd hardly ever seen a touring folk band," she says.

"I remember Ossian coming to Stornoway and they made a big impression but bands didn't seem to get to the islands the way they do now, so I hadn't a clue. I couldn't even use a microphone properly on the first gig I did with Seelyhoo. Then the second gig was at Celtic Connections and I was so sick that I went onstage feeling awful but you just had to get on with it.

"Immediately after that we did the annual folk tour of Germany, a big tour of concert venues. It was quite an experience and a steep learning curve but after that mad initiation I was hooked. I completed my degree but I didn't go back to uni for my honours year. I just wanted to sing."

Seelyhoo recorded two albums in the four years they were together and towards the end of that period Mackenzie met Ã" hEadhra, who was working with another band, Anam.

Their singer, another Orcadian, Amy Leonard, was growing tired of living out of a suitcase as Anam toured the world and she eventually left for a more settled lifestyle, so for a brief spell Mackenzie sang with both bands.

"I'd just got together with Brian when he said he had to go off to Australia and I don't remember where else for three months and so when I joined Anam I thought I'd be touring the world too," says Mackenzie, who by this time had moved to Edinburgh and become part of the capital's thriving folk scene of the 1990s.

"I did get to go to Japan, which was amazing, but not long after that the record company started cutting its budgets and I think I saw the less exciting side of touring with Anam but it was still great fun and I really enjoyed the music."

Back at home, she and sisters Gillian and Eilidh recreated the back seat of the car singalongs that used to soundtrack family holidays and made them into something altogether more artistic on a trio of albums, one of which, Camhanach, featured production by the late Michael Marra and another of which led to Mackenzie making a solo album of her own songs, Elevate, for Linn Records.

"I wasn't looking to start a solo career," she says. "I'd been writing songs since I was fifteen and I wanted to make a demo of them, so I asked Calum Malcolm, who's a great producer, if he'd help and he got the deal with Linn.

"It's just as well I wasn't hoping to become a star because it was a long, drawn out process. It took five years. In fact, when I went into the studio at first I had no children and by the time the album was released I had two."

Cruinn came about when Ã" hEadhra said one day that they should form a group that concentrated on singing, as opposed to mixing songs in with high octane instrumentals as often happens in folk bands. James Graham from Lochinver in Sutherland, a former Young Scottish Traditional Musician of the Year, and Rachel Walker from Kinlochewe in Wester Ross, whose singing Mackenzie and Ã" hEadhra had long admired, were the obvious choices. Two phone calls later, as Mackenzie says, Ã" hEadhra's dream group was formed.

At the forthcoming Tradfest celebration of traditional arts in Edinburgh the quartet will represent the Gaelic tradition in a programme that also includes Swedish, American, English and Irish singers and musicians.

The BBC Folk Awards recognition has given them a boost but this most laid-back and unshowbiz-like of troupes is likely to remain unaffected by the attention. Wednesday's trip to Cardiff for the awards ceremony was being viewed in advance as a belated staff Christmas outing.

"Brian and I have been working as a duo for the first time recently and we're hoping to do more of that," says Mackenzie. "And the great thing about Cruinn is that we can all go away and do things on our own like that and then pick up where we left off.

"There'll be a third album to work on before too long but we haven't set a deadline. It's a really stress-free situation to work in. We all have similar attitudes and we're all quite relaxed but when the time comes to record or perform, we're professional."

Tradfest runs in various Edinburgh venues from Wednesday, April 29 to Sunday, May 10. Cruinn appear at the Scottish Storytelling Centre on Saturday, May 9