MENTION JONI MITCHELL and chances are you'll think of her extraordinary voice on such classic songs as Big Yellow Taxi, Both Sides Now, The Last Time I Saw Richard or A Case of You, a voice acrobatic in its ability to soar and dive, to shift pitch and timbre in the middle of a phrase, to rush ahead and cut back on itself, to constantly surprise. And in her songwriting this vocal virtuosity is matched by a playful linguistic skill, manifested in a profusion of vivid, incisive images, caught on the wing and precisely fixed with amazingly few words. Inevitably, the range of her voice has narrowed with time but, as last year's album Shine reaffirmed, Mitchell is much more than a voice, and much more than a lyricist: hers is a formidable musical intelligence.

She is also a painter, and has often referred to the importance for her of painting. Her exhibition Green Flag Song, which features as part of this year's Galway Arts Festival, comes with a well known quote from her: "I have often thought of myself as a painter derailed by circumstances." As it happens, the work in the exhibition is painterly but employs photographic techniques, a first for her.

As she sees it, it both is and isn't a departure. "It's new in that it's media art, all the way, it's not painting. But some years back I did a retrospective of my paintings at Saskatoon and when we were going through the work I could see that it was made up of a lot of different periods. I never went along with the idea of finding a style and sticking to it. I was always restless in changing style, and because I have this other job, some of the periods were very brief, maybe just three or four paintings. But there was a continuity in terms of my eye for an image, and in terms of palette, and that applies to Green Flag Song as well." She knew she wanted to be a figurative painter: "I'm kind of somewhere between Van Gogh and Gauguin. I like Picasso and I could follow him into the Cubist thing, but I didn't really want to go there myself." Her work often features on her album covers, including a self-portrait on Turbulent Indigo which is a version of Van Gogh's famous self-portrait with a bandaged ear. Generally, though, there is a mellowness to her paintings that sets them apart from the often emotionally fraught themes of her music, though music and visual art share common roots for her.

"The thing that started me painting originally was seeing Bambi when I was about nine. I was incredibly disturbed by the forest fire that killed Bambi's mother, and that distress gave me the impulse to create something, as a way of dealing with it." It was distress on another level that spurred her into writing and recording Shine when it was more or less presumed that she'd retired from making music, and also led her to make the works in Green Flag Song. "As the world accelerated in its insanities, I thought I just had to do something. I've written about these environmental and political issues before, but 20 or 30 years ago, in songs that were ahead of their time and that have dropped out of sight." Shine is a surprisingly meditative and quietly engaging album, but it is not resigned or elegiac. There's a fierce, subdued anger rumbling along under the surface as the "War on Terror", environmental despoliation and other contemporary ills come into focus. She has a particular loathing, for example, for our infatuation with technological gadgetry. The final track is a subtly amended version of Rudyard Kipling's If, and the cumulative message that emerges is that we can't turn our backs and walk away from responsibility for the world we inhabit.

The images that make up the triptychs in Green Flag Song were gathered from the television screen. They are drawn mostly from news footage - including scenes from Iraq and Afghanistan - but incorporate other material as well, some of it surprising in context. "There are Busby Berkeley dancers among the images of war," Mitchell notes. One night she happened on a screening of Berkeley's musical Gold Diggers of 1937 when her television went on the blink, giving the images a distanced, hallucinatory quality she liked. So she set out to emulate the effect, taking photographs of the screen with a series of disposable cameras.

The juxtaposition of images has a rationale. "What struck me was that Busby Berkeley actually dealt with current issues, in the same way that Chaplan or Groucho Marx did, with a certain lightness. They addressed the problems but people left the theatre happy." Although Green Flag Song is violent and cacophonous, it has a dreamlike quality by virtue of the way the images are processed and translated, Mitchell observes, into un-naturalistic shades of "green, pink and magenta". She is a keen and reputedly very good dancer and, during the time she was working on the album and the exhibition, she was also engaged in devising a ballet, The Fiddle and The Drum, based on her music, for the Alberta Ballet, with choreographer Jean Grande-Maître. "I'd never worked as hard in my life."

She was born Roberta Joan Anderson in Fort Macleod in Alberta in 1943, though her family soon moved, first to North Battleford and then to Saskatoon in Saskatchewan. Her father was of Norwegian stock though, even given her own distinctly Scandinavian appearance, she suspects the lineage may be more complicated, because he is said to have taken on the name Anderson on Ellis Island. Her mother's maiden name was McKee so, she notes, she has Scottish and Irish blood as well. She came through a bout of polio aged nine which was also, remarkably, the year she started to smoke.

She remains an enthusiastic advocate of smoking. "How did Ireland give up so easily on smoking?" she asks. "People are going to die of butter, or alcohol, or something. Why pick on cigarettes? I really couldn't have gotten through life without them, because I have a certain kind of nervous temperament and they calm me. I also couldn't have done as much, because smoking helps me to focus. I was sitting on a terrace in LA and this guy complains about the smell of smoke. I hadn't even lit up. Then I overheard him complaining that nobody can concentrate anymore and I said, 'Yeah, it's because they're not smoking'. The world is so full of ex-smokers, I don't know how anyone gets anything done."

She was the girl who was good at art in class, though she also liked music, growing up with a love of classical music. Already playing guitar and singing folk songs in local cafes, she was disappointed at the scope of activity when she went to the school of art in Saskatoon, and moved on to Toronto. Finding she was pregnant by her ex-boyfriend, she precipitately married a folk musician, Chuck Mitchell, and gave her daughter up for adoption. The marriage broke up within 18 months. She moved on to New York and, in 1968, to Los Angeles. Subsequently she's been based between her homes there and, since 1971, northwest of Vancouver in British Columbia.

She hates being stereotyped as a folk singer, a girl with a guitar. "That's not difficult, I was a pro within six months," she notes acidly. In fact she never really was a folk singer. An exceptionally restless spirit, she consistently moved on and broadened her range and abilities from the very beginning. From her first album Song to a Seagull in 1968, her popularity grew progressively to the ecstatic reception accorded Court and Spark in 1974.

Her second marriage, to musician Larry Klein, broke up in 1994. Around about then, she began to speak openly about her daughter, and they were reunited in 1997, when she found she was also a grandmother, a role she reportedly embraced with enthusiasm. By the turn of the century, she was having problems with her voice. "I'd go to hit a note and there was nothing there," as she put it, simply. A compressed larynx and nodes on her vocal chords were diagnosed. It's not her favourite album by any means, but Both Sides Now from 2002, with orchestrated versions of standards, shows that she managed to adjust her singing to the constraints of her voice brilliantly.

When asked whether there's a section of her repertoire that she feels has been particularly neglected, she responds with alacrity: "Pretty much everything after Court and Spark." That is, everything after 1974.

While Court and Spark was enormously successful, it's worth noting that it introduced an integral element of jazz into her musical language, an element that has been consolidated and developed subsequently. Despite various moments of stylistic fusion, popular musical genres tend to shy nervously away from jazz. And Mitchell says she was specifically warned off when Charles Mingus got in touch with her to work on a project that eventually became Mingus. "They wouldn't even tell me he was looking for me, and then they told me that I'd never get on radio again if I went ahead with it. And they were right." She wasn't a Mingus enthusiast to begin with, and the resultant album is not her favourite. "But I don't regret it, I'd do it again for all that I learned from the experience. It was invaluable." She continued to work with many fine jazz musicians, including the late Jaco Pastorious, Wayne Shorter and Herbie Hancock, whose recent album River: The Joni Letters is a particularly good tribute. Among the singers on the album, she singles out Tina Turner as making a fine job of Edith and the Kingpin: "When you sing you've got to bring the story to life, and she does that." She's also praised Cassandra Wilson and Diana Krall as interpreters of her songs.

In terms of relative neglect she does mention specifically the albums she made with David Geffen's own label throughout the 1980s (now available in a four-disc boxed set). Her second Geffen project Dog Eat Dog saw her make controversial use of synthesizers to produce a radically different kind of sound, and it was also controversial in lambasting televangelists and what she saw as a drift to the religious right in American politics.

"The churches came after me, they attacked me, though the Episcopalian Church, which I've seen described as the only Christian Church in America that actually uses its head, wrote me a letter of congratulation. I could see the separation of church and state was disappearing, and that worried me. I think it was prophetic in terms of what's happened since, but the press was dismissive at the time. I think Time magazine called the album 'sophomoric' - as though you're supposed to stop thinking about this stuff, to leave your political passion behind when you finish college."

Her views on the popular music industry are well known - she angrily denounced it in 2002 when she said she was opting out for good - and if you were cynical you might say that she is merely offsetting responsibility for the fact that the earlier phase of her recording career remains by far the most successful, that she has never matched Blue and the absolute peak of Court and Spark in terms of sales and popularity. But look at the extraordinary breadth and richness of what she's done since (Hejira and Turbulent Indigo to take two readily accessible examples) and you will have to concede that she has a point.

It's hard to think of a contemporary popular musician with a comparable back catalogue or level of influence among her peers. But it seems that the record industry was and is ill-equipped to cope with a musician who follows a path of genuine musical development over time. The industry attitude, she argues, is that definitions of genre and personal style are inflexibly fixed, and if you cross the boundary you'll be punished. It's also true, though, that much of her work is complex by the standards of popular music. It continually challenges the listener, and the industry, together with the bulk of the potential audience, is conditioned to smooth, effortless consumption.

"I know I went the long distance route," Mitchell observes. "My interest in both art and music is eclectic, and it takes a long time to synthesize something out of that." She reckons the emotional expressiveness of her music is also a problem in certain respects. "I hate the way the songs on Blue are always described as 'confessional'. They're not confessional, they're intimate, which is something different. A lot of men, particularly, are uncomfortable with that, with the personal, the emotional. Hejira found an audience among gay men, I think because they are more open to the things it deals with. And I've always had a large Afro-American following, I think for the same reason."

She is known to be outspoken, and unashamedly sure of her own worth and abilities. Though it might seem odd to say it, her very directness and willingness to talk leave her curiously open and defenceless. She simply doesn't do guile or diplomacy. It depresses her, she says, to see how passive people have become. When Green Flag Song was on view in Los Angeles she spoke to young people who'd seen it, and what amazed her was their assumption that they couldn't change anything about the way the world was organised. "That to me is tragic."

Writing in The New York Times about Sheila Weller's book Girls Like Us, which profiles Mitchell, Carole King and Carley Simon (and was written without Mitchell's co-operation or approval), Stephen Holden made an interesting point about the music of the three. While in the 1970s all were seen as models of female sexual emancipation, their work reflects an enduring belief in romantic love. Such an attitude, he suggests, predates Madonna's "cold heat", in which bodies are purely machines and emotions are reduced to a matter of ironic posturing. In a way the distinction drawn is between modernity and postmodernity. Madonna, the post-modern performer believes she's dealing with a commodified world of empty representations to be manipulated for profit. It is clear that Mitchell, on the other hand, still presumes her music has some purchase on, and responsibility to, reality. That's unlikely to change, even though, as she puts it: "I still believe the pen is mightier than the sword, but I'm beginning to wonder."

Green Flag Song, works by Joni Mitchell, is at the Festival Box Office, Merchants Road, Galway, until July 27th