In December 1922 Dublin formally became the capital of the new Irish Free State. Ten months earlier, when James Joyce's Ulysses was published in Paris, Dublin had already become the capital of the universe. Joyce told Arthur Power, "For myself, I always write about Dublin, because if I can get to the heart of Dublin I can get to the heart of all the cities of the world. In the particular is contained the universal."

Implied in this was something much more radical than a parochial defiance of the great imperial cities. Joyce was showing in Ulysses that, in the modern world, universal experiences were to be found not with gods or heroes but in the particulars of everyday urban lives. His book is a dazzling compendium of the mundane, from the graveyard to the maternity hospital, from bodily fluids to global trade, from dirty minds to elevated thoughts, from newspapers to popular song, from mockery and infidelity to kindness and love.

TS Eliot told Virginia Woolf in September 1922 that Ulysses "destroyed the whole of the 19th century" and in 1923 called it "the most important expression which the present age has found". These grand claims had a great deal of truth: Joyce's "stream of consciousness", expressing the inner workings of the mind, his successive pastiches of English styles, and his replacement of a single narrative with the random intersections of multiple characters on a single day all had profound consequences for 20th-century art.

But if Ulysses were simply a technical experiment it would be unreadable. The real shock of the book – and its most profound statement – lay in its use of these prodigious techniques to express not wonders but the physical, emotional and intellectual lives of lower middle-class people in a marginal place.

At the end of A Portrait of the Artist As a Young Man Stephen Dedalus's mother had prayed that he would find "what the heart is and what it feels", and at its core this is what happens in Ulysses. His mother dead and his father useless, Stephen finds, as it were, a new father in the figure of a quiet, unprepossessing advertising canvasser, Leopold Bloom.

Bloom and his unfaithful wife, Molly, act as Joyce’s versions of Homer’s royal hero and heroine, but the parallel is ironic. They are hurt, flawed people, and Bloom’s day in particular is full of petty humiliations. Yet they have more vivid and detailed inner lives than any characters in fiction had ever been afforded before.

Ulysses may be mock-heroic, but it is also deeply forgiving and staunchly democratic. We get to know all of Leopold and Molly's dirty secrets, bad thoughts and grubby fantasies, but instead of being lessened by this exposure they are magnified to an astonishing complexity.

This generosity extends to Joyce’s implied vision of what it might mean to be Irish. The three main characters have lived, or have their roots, elsewhere. Bloom, attacked by an anti-Semitic nationalist, insists that his nation is Ireland. Asked what a nation is, he replies, “The same people living in the same place.” And then, being Irish and Jewish, he adds, “Or also living in different places.”

It was not the official definition of Irishness, but, as with so much else in Ulysses, it was a truer one.