There is no excuse for what happened to Flann O'Brien. He died in 1966 at 55, a bar-stool celebrity in a dirty old town. Dublin raised a glass in his memory while secretly toasting the poetic squalor of his demise. His friend Patrick Kavanagh, equally gifted and equally betrayed, would expire nearly as badly three years later. Few artists lived or died well in Ireland back then.

Twenty-seven years earlier, the likes of Graham Greene and James Joyce had heralded the young O'Brien for a debut novel called At Swim-Two-Birds. Beginning in 1967, with the posthumous publication of The Third Policeman and the translation from Irish of The Poor Mouth, his reputation began to rise again, to the point where the likes of novelist Edna O'Brien now assign him a place in the pantheon of Irish literary immortals, alongside Joyce and Samuel Beckett.

But in between sensational debut and posthumous vindication lies a botched career and a squandered gift. Chronology is important to understanding both this possibly harsh verdict and the impatience about to be shown with the mildly amusing The Various Lives of Keats and Chapman: Including "The Brother". The collection not only misrepresents Brian O'Nolan, as Flann O'Brien was known to family and friends, but presents him as a sort of antecedent to Brendan Behan or Malachy McCourt: a stage Irishman for export, blowzy and loud and too eager to please. O'Brien never behaved this way, even in his decline.

Here is the chronology. In 1939, Brian O'Nolan published At Swim-Two-Birds under the name Flann O'Brien. He received an advance of £30 from Longmans in London, based on the enthusiastic report of their reader, Graham Greene. The novel racked up 244 sales before the remaining copies were destroyed by a Luftwaffe bombing of a warehouse. As such, the edition was literally killed by the war.

Undaunted, O'Nolan sent Longmans another manuscript in 1940. It was called The Third Policeman, and the harried publisher, finding the story odd and disturbing, declined. The author subsequently buried the book in a cupboard, inventing absurd tales to account for its disappearance.

In 1941, a Dublin house issued An Beal Bocht (The Poor Mouth), a wicked satire of Gaelic-culture chauvinism. For this venture, O'Nolan, who was raised in an Irish-speaking household, employed the name Myles na Gopaleen, the moniker he had introduced in a column he had lately begun to write for the country's leading newspaper, the Irish Times. Now all of 30, with a government day job to attend as well, he was still 12 months away from having his first play, Faustus Kelly, put on by the Abbey Theatre.

A more auspicious, if bumpy, start would be hard to imagine. Lurking among those early successes, though, were the shadows that would soon envelop him. The lesser shadow, ironically, was the Times column Cruiskeen Lawn, and the persona of Myles na Gopaleen. By 1943, O'Nolan had switched from writing the pieces in Irish to English, and was amusing and exasperating vastly larger audiences than he had found for either At Swim-Two-Birds or, obviously, the novel in the cupboard. He liked the attention and acclaim.

O'Nolan would continue to produce the column until the bitter end, in addition to scribbling freelance journalism for other publications under a bewildering array of pseudonyms. Cruiskeen Lawn, from which most of The Various Lives of Keats and Chapman is drawn, is one of the great newspaper columns of the last century. That said, the writing was brilliant for maybe five years and decent for another five. For the remaining years, O'Nolan ground out copy with a grim determination that could have been mistaken for spite. He was then a heavy drinker who had allowed narrow, puritanical Dublin to anoint him a favourite son – the worst fate for an Irish artist of the period. Joyce and Beckett fled to the continent for a reason.

But the larger shadow was probably cast by The Third Policeman. Not the book itself, which is a rough masterpiece, a parable about power and identity worthy of Franz Kafka. Rather, it was the failure of O'Nolan's confidence in the manuscript, evidenced by his decision to accept the verdict of just a single publisher, that may have banished him from the natural light of his own gift. He certainly never attempted anything so ambitious or risky again.

First of all, The Various Lives of Keats and Chapman isn't by Flann O'Brien. Myles na Gopaleen wrote two-thirds of the text and he was, in the author's Manichean mind, a distinct creation. The space constraints of journalism left Myles in a perpetual rush to line up the gags and shaggy-dog stories, leading to an over-reliance on the workhorse of Irish humour: the pun.

"It is perhaps only in Ireland that the pun is given is proper due," the novelist Jamie O'Neill observes in his introduction to this first North American edition of a book originally published in 1976. Selections from Cruiskeen Lawn are devoted exclusively to plays on words. The characters Keats and Chapman, regulars in the column, roam the planet and take up every variety of labour all for the sake of a deflating pun.

"For once I admire your mise en Seine," Keats tells his friend in Paris, having watched Chapman's theatre backdrops fall into the river. Or this, while studying an embalmed corpse from ancient Greece. " 'He doesn't look half his age,' he said. 'He's very well preserved.' " Then there is The Brother, a 50-page monologue first cobbled together by the actor Eamon Morrisey in 1974. It mingles column excerpts concerned with another na Gopaleen character, a Dublin pub-snug philosopher and his outlandish off-stage "brother," along with passages from the novels. Funny in a theatre, the monologue falls mostly flat on the page.

Worse still, by lifting bits from The Third Policeman, the monologue compounds O'Nolan's original error. In his twilight, Flann O'Brien published a mediocre novel called The Dalkey Archive. Buried in the text were inspired comedic riffs that were later revealed to have been cannibalized from the suppressed manuscript. To find fragments of the major novel O'Brien failed to see to publication in 1940 cast once more in a minor literary setting is to break the hearts of his admirers all over again. There is no excuse for what happened to Flann O'Brien – or maybe for what Brian O'Nolan did to himself.