IT HAS to be one of the most instantly recognisable images in classical music. Benign and bespectacled, surrounded by its halo of tousled, Byronesque curls, the face of Franz Schubert has gazed mildly out at the world since his friend Wilhelm August Rieder painted his portrait in watercolour in Vienna in 1825. But Schubert's face is as deceptive as his music. Beneath the surface calm and conventional elegance there are mysterious depths where almost anything can happen.

Though it carries echoes of Mozart and Beethoven and whispers of Liszt and Wagner, the sound-world of Schubert is unique and ambivalent and, in many ways, owes little to anybody. And although this year's bi-centenary of his birth will undoubtedly inspire another flood of biographical research, Schubert's short life stubbornly refuses to give up its secrets; in fact, the more we learn about Schubert, the less we seem to really know.

The music is the easy part. "I love the bittersweet quality of it," says William Howard, founder member of the Schubert Ensemble, which embarks on a bicentenary tour of Ireland with Music Network on Saturday. "Although it always seems very optimistic and lyrical, there's always an underlying darkness and restlessness - which is a combination that I can identify with. I think one of my favourite pieces by any composer is the second piano trio, which we'll be playing on this tour. It's a pretty powerful work with amazing extremes of emotion."

According to the National Symphony Orchestra's principal conductor Kasper de Roo, who will wield the baton at the opening concert of the National Concert Hall's bicentenary weekend on January 31st, "there is something so special in Schubert's music that it doesn't compare with anything else in music literature at all. Schumann wrote of the Ninth Symphony that it seems to come from a country we all know, but we don't know exactly where it is. He also writes about the first movement, that it starts in a very special way - that suddenly we are in the tempo without knowing how we got there. And Stravinsky always said, when he was asked about Schubert, well, after listening to his music I have the feeling that I had a wonderful dream'."

DREAMLIKE, mysterious, overwhelmingly beautiful and extraordinarily precise; in Schubert's music a romantic heart beats inside a classical body. Who else could recreate with spine tingling accuracy the fevered hallucinations of a dying child (Erlkonig), the sound of a creaky spinning-Wheel (Gretchen am Spinnrade), the stab of pain as a thorn pricks an unsuspecting finger (Heidenroslein)? Who else could so lucidly express the inexpressible; the departure of the soul for paradise (Auf dem Wasser zu singen), or the inexorable progress of life towards death - (Winterreise) -and all in the form of tunes so apparently artless that a child could whistle them after one hearing? One musician once wrote of him that he could "catch the sound of starlight"; for the pianist Artur Schnabel, Schubert was the composer who was closest to God.

So much for Schubert the composer. Schubert the man is an altogether more problematic proposition. Of his 31-year lifespan there remains little by way of primary source material; a handful of diary entries, a few poems, a couple of dozen letters. The memoirs published after his death by various friends are at best hazy, at worst downright inaccurate. In a biographical study published last year (Franz Schubert: a biography, OUP, .£25 in UK), Elizabeth Norman McKay is forced to get into an analysis of the finer points of Schubert's letter-writing style, circa 1822, in an attempt to prove that "something momentous" had recently happened to him. "In all his letters to friends until now, he had always used `recht' in qualifying the state of his own health: `recht gut `recht gesund', `recht wohl'. Why, one may wonder, does he now modify the qualification to only `ziemlich gut'; from `very well' to `quite well'?"

This is rarefied territory indeed: yet in the course of her book McKay suggests that not only was Schubert a manic depressive who aggravated both his mental problems and the syphillis which was diagnosed in 1823 by persistent, excessive drinking and smoking. He may, she suggests, have smoked opium on a fairly regular basis; he may or may not have been homosexual; he was certainly as unwholesome in his personal habits as he was unreliable in his social ones.

Despite McKay's incredibly painstaking detective work, few of these suggestions can be proved one way or the other, but if they don't exactly make him an attractive proposition for retrospective celebrity status, they do make him sympathetic, in an odd kind of way. In his lyrical little study of Schubert in the Classic FM Lifelines series (Pavilion Books, £4.99 in UK), Stephen Jackson offers his life and work as the last word in modernity. "We find ourselves in an era as much preoccupied as the early 19th century was with the spread of sexual disease and the violation of innocence; a pre-millennial circumspection, it seems, for which Schubert's triumph over his fate and his own meagre life seems increasingly to the point.

VICTIM or vagabond, there can be no doubt about the quality and quantity of Schubert's output. After his death it took most of the 19th century to sort it out and add it up; 600 Lieder, eight symphonies, 22 piano sonatas, six Masses, 15 operas, 35 assorted chamber works. "Yes, and not many people know that he wrote 400 dances for piano solo - waltzes, polonaises, ecossaises and the like, arranged in cycles for performance. They're a revelation," says William Howard.

"In his late works there is an extraordinary maturity - there's something approaching the wisdom of an old man in the music that he wrote towards the end of his life. That last two years was one of the most astonishing outpourings of work of any composer - I mean, it's hard to work out how he even had the time to write so many pieces. It's not as if he was dashing them off - they're well crafted - but it's as if he was working to a kind of deadline."

Kasper de Roo agrees. "The Eighth and Ninth symphonies are splendid works - the Unfinished is so finished that you just can't believe it. His music is popular, but it's never populist; it's refined, and of very high quality, and yet as a person we know that sometime he would drink a little bit too much and so on. If you compare his music with Beethoven's, in Beethoven you get the feeling that he is interested in the construction of the music; with Schubert it just flows out of him, and there's something very mysterious about it. He opens a gate to which we don't have a key."