Christine Tobin stops mid-sentence and tells herself to put her shovel away and stop digging. The Dublin-born singer has just been waxing lyrical about the power of sad songs to make both singer and listener feel better when she realises she might be setting herself up for inclusion in the miserable-git school of performer.

She's far from miserable. It's true that, with arguably the most distinctively alluring voice on the British jazz scene, Tobin can sound like the roof's just fallen in on her world and then some. But that's only when the song demands it. On her latest album, Secret Life of a Girl, Tobin changes character like a jobbing actor - now the worldly, sardonic observer, now the 10-year-old Camille, all mischief and wide-eyed wonder - and in conversation she's fun, effusive and full of enthusiasm for the latest gig she's been to and the next stop, whatever it'll be, on her constant voyage of musical discovery.

The night before, she'd been to see Herbie Hancock at London Jazz Festival, and she's still loving what she heard from a musician who played a part in her conversion to jazz, which came when Tobin heard Joni Mitchell's Mingus album in her late teens. Here were brilliantly crafted lyrics set to tunes written, mostly, by one of jazz's greatest composers and played by a dream band, including Hancock, saxophonist Wayne Shorter and bass guitar genius Jaco Pastorius.

"For me at the time, it was the unpredictability of the music, the way it twisted and turned, and the great sounds that the musicians created, and on top of that there were Joni's words," she says. "I already loved Bob Dylan's lyrics, and I still do, but this was different. There was a style to Joni's writing that just intrigued me and made me want to sing those songs."

Before Mingus, Tobin had almost switched off from playing music. As a child growing up in Dublin she'd played Irish music, doing the rounds of socials and charity concerts with her older sister as an accordion duo. Then at the age of 11, she auditioned for a stage version of The Good Old Days, the television programme that brought music hall and a dictionary-swallowing master of ceremonies into our homes for a good many years, and got the part, singing and playing the accordion. She loved being onstage and might even then have harboured notions of becoming a professional singer, but the music her peers were listening to - the pop music of the day - didn't really appeal.

"I liked the stuff my sister, who's 10 years older than me, was listening to," she says. "She was into Jimi Hendrix and Laura Nyro and Dylan, which was a bit kind of grown-up for an early teenager to be thinking about playing. So I didn't really do much until my late teens, when I heard Mingus and decided to become a jazz singer."

Jazz quickly became an obsession as she acted on tip-offs and followed the time-honoured discovery by association process, going back to Charlie Parker and happening across Sarah Vaughan and - still a big favourite - Billie Holiday, then learning as many jazz standards as possible.

"I loved Billie Holiday the most from the start," she says. "It was the emotional quality, and the sense of truth. It's a mixture of an amazing humility, human frailty, yet absolute command as well, and she still sounds really contemporary, even the early recordings, with that extraordinary timing and phrasing."

After singing around Dublin for a year or so, Tobin decided to broaden her horizons, so in 1987 she moved to London. She took the jazz course at the Guildhall School of Music, where she made full use of the record library, digging into Miles Davis's whole back catalogue with especial relish, and where she particularly enjoyed trumpeter and writer Ian Carr's jazz history class and being able to trace the music's development. Inevitably, being around the Guildhall, there were opportunities to sing and Tobin joined pianist Simon Purcell's band, where, as well as singing the pianist's own arrangements of jazz standards, she had another door open for her, as a lyricist.

"Up to this point, I'd seen myself as an interpreter, I suppose, but Simon really pushed me to write," she says. "He was really encouraging and, although I found it pretty difficult, I wrote some words to tunes he'd written. Then I got a bit overwhelmed and I was a bit confused as to which direction I should take. You know, should I continue singing standards, which is what jazz singers mainly do, or should I do my own stuff?"

In the end, she chose to do neither. Instead, she took a course in anthropology at Goldsmiths College. "I think I needed to step back from it all and here was something that was all about human beings and communication that didn't involve getting up on a stage," she says. "It was a really interesting subject and going back to college turned out to be a good move because within two years of being away from music, I could really see my way forward."

Enter, at this point, saxophonist Tim Garland and award-winning Dundee-born poet and guitarist Don Paterson, who were putting together the folk-jazz group Lammas. They already had the bulk of their first album recorded when Garland approached Tobin and asked if she might sing on one track, the group's own arrangement of the traditional song Black is the Colour (or Black Hair as it became listed on the CD).

"It was funny because when I got to London, everyone presumed that, being from Ireland, I was coming from a traditional music background," says Tobin. "But apart from playing the accordion as a child, I'd had no real traditional music experience. In fact, the first time I heard Black Hair was when Tim left a recording of it on my answerphone so that I could learn it."

Nothing daunted, Tobin made the recording, joined the band and while Lammas made a series of criminally overlooked albums through the 1990s (with Tobin, for a non-folkie, providing some great takes on Robert Burns etc), she was able to establish herself simultaneously as a singer in her own right. The seven albums she's made for the Babel label, beginning with Aililiu in 1995, chart her progression into an assured and inspired songsmith who can also give whole new lives to songs by writers including Leonard Cohen, Bob Dylan, Milton Nascimento, Cole Porter and, on Secret Life of a Girl, Rufus Wainwright.

"I still love standards and I know that not being easily categorised - is she a singer; is she a songwriter? - can make life difficult," she says. "But music, for me, should take people somewhere different from their everyday lives.

"I love the storytelling aspect of songwriting and the whole world of words, which is something I got from working with Don Paterson and other poets like Michael Donaghy and Eva Salzman. But in the end it's all about communicating the emotions and leaving people feeling that they've had a meaningful experience."

Christine Tobin and her band play at The Blue Lamp, Aberdeen, tonight and at the Tron Theatre, Glasgow, on Wednesday, January 28.