Of all the composers in RTÉ's American Originals Weekend, which begins today, it was Charles Ives who made the most significant contribution to modern music. It's perhaps not what you would have expected from an insurance broker, writes Michael Dervan.

Charles Ives, who plays a key part in RTÉ's American Originals Weekend, which begins today, was in many ways the most unlikely of people to end up becoming not only the United States' greatest composer but also the first to break away definitively from the revered inheritance of European tradition.

He was born into small-town America in 1874, became a salaried organist at the age of 14 (the youngest in Connecticut), studied music at Yale, began his First Symphony in 1898, with more than a nod towards Dvorak, and worked as an organist and choirmaster in New Jersey and New York. But, as he put it himself, in 1902 he "resigned as a nice organist and gave up music".

He made his living as a successful businessman - he was a partner in the insurance company Ives & Myrick - and composing became a spare-time activity. By 1920 he had written most of his music and was increasingly devoting himself to revision and promotion. He started no new pieces after 1927.

He had, of course, had the enormous benefit of a musical father, a bandmaster in Danbury, Connecticut, who showed a strong experimental streak. "I couldn't have been more than 10 years old," he recalled, "when he would occasionally have us sing, for instance, a tune like The Swanee River in the key of E flat but play the accompaniment in the key of C. This was to stretch our ears and strengthen our musical minds."

Ives senior also experimented by having bands march past each other in the open air playing different tunes, just to see what the sonic mixture would sound like. "Father had a kind of natural interest in sounds of every kind, everywhere, known or unknown, measured 'as such' or not."

The young composer inherited his father's taste. Some of his surviving church music from the 1890s is extraordinarily daring in its use of dissonance. But the embrace of dissonance was only one of the ways in which Ives would anticipate musical developments that would later become widespread.

Polytonality, polyrhythm, collage and raw vernacular quotations all make early appearance in Ives's work. "There can be nothing exclusive in a substantial art," he said, "It comes directly out of the heart of experience of life and thinking about and living life. My work in music helped my business and work in business helped my music."

His experience of life was full of popular tunes, hymns, local pride and lore about the American Civil War, all of which found place in his music. And as he was his father's son, the references were not neatly separated out but often jostled together, like hazy memories that, although hard to keep in focus for long, stir deep, half-forgotten feelings.

The complexity and dislocations are sometimes staggering, and the expressive point, even when naive, often potent. The difficulties for performers are great, which helps to explain why performances of some of the major works are rare but special occasions - Sunday's performance of the Fourth Symphony will be an Irish premiere. And the composer didn't help by working and reworking his material, from one piece to another, and frequently adding new layers to old compositions, so that definitive dating is often a musicologist's nightmare.

It has even been claimed that his working practices challenged the notion of pieces of music being concrete, fixed entities. The composer Elliott Carter, who regularly visited Ives at home, gives credence to this.

Ives used to play favourite bits from his pieces but played them differently every time, "sometimes changing the harmonies, the dynamic scheme, the degree of dissonance, the pace", according to Carter.

Ives explained this by saying that the printed score was "only a general indication" and that performers should recreate the work for themselves. Carter was always impressed with Ives's playing. "Being a good sturdy Yankee with plenty of vitality," he wrote, "he poured lots of pep, salty humour and good spirits into everything he did."

There have also been suggestions that Ives engaged in an amount of tinkering in order to stake his claim for priority for certain musical developments. But these challenges have been scotched by modern techniques in the study of handwriting and the dating of paper.

The American Originals programme includes three works by Ives. The earliest, the First String Quartet (1898-1902) is based entirely on hymn tunes and has both a title, From The Salvation Army, and a subtitle, A Revival Service. It's a fair indication of the general fate of Ives's music that the work had to wait until 1957, three years after his death, for a complete performance.

The Second Piano Sonata, Concord, Mass., 1840-1860, is, according to the composer's 1920s note, an "impression of the spirit of transcendentalism that is associated in the minds of many with Concord, Mass., of over half a century ago" - the movements bear the titles Emerson, Hawthorne, The Alcotts and Thoreau.

It's the work that launched Ives to celebrity status when John Kirkpatrick gave its New York premiere, in 1939. "This sonata," wrote Lawrence Gilman in the New York Herald Tribune, " is exceptionally great music - it is, indeed, the greatest music composed by an American and the most deeply and essentially American in impulse and implication."

The piece was written in about 1915 and published, at the composer's expense, in 1920.

Ives had originally planned a set of essays to be published in the same volume, but music and text, under the title Essays Before A Sonata, were issued separately. "These prefatory essays," he explained, "were written by the composer for those who can't stand his music - and the music for those who can't stand his essays; to those who can't stand either, the whole is respectfully dedicated."

The Fourth Symphony, which was mostly composed between 1910 and 1916, had to wait for a complete hearing until 1965, when the 83-year-old Leopold Stokowski played it in New York with his American Symphony Orchestra.

It's the quintessential multilayered Ives experience. The writer Henry Bellamann, an Ives apologist, explained its aesthetic programme as being about "the searching questions of What? and Why? which the spirit of man asks of life. This is particularly the sense of the prelude. The three succeeding movements are the diverse answers in which existence replies".

Ives later declared the last movement to be "the best, compared with the other movements, or for that matter with any other thing I've done". He described it as "an apotheosis of the preceding content, in terms that have something to do with the reality of existence and its religious experience".

Recognition came late to Ives. Some of it was public - he won a Pulitzer Prize in 1947 for his Third Symphony, a work that he completed in 1904 - and some was private, as in a note found among Schoenberg's papers after his death, in 1951. "There is a great Man living in this Country - a composer. He has solved the problem of how to preserve one's self and to learn. He responds to negligence by contempt. He is not forced to accept praise or blame. His name is Ives."

And the recognition that matters most, the performance of his music, has continued to gather pace.

• RTÉ American Originals Weekend, also featuring music by John Adams, Samuel Barber, Leonard Bernstein, John Cage, Elliott Carter, Aaron Copland, Henry Cowell, George Gershwin, Philip Glass, Louis Moreau Gottschalk, Conlon Nancarrow, Steve Reich, Christopher Rouse, Carl Ruggles and Frank Zappa, is at the National Concert Hall and National Gallery of Ireland, Dublin, until Sunday. Details from 01-2082617, or see www.rte.ie/music/americanweekend.html