They say a picture is worth a thousand words, but this blurry black and white image breathes just one: mystery. The girl looks directly to camera. She’s wearing pearls and a white dress, creamy symbols of innocence, but the overwhelming impression is black: a mass of black hair, frayed at the edges; a black mouth, caught in an expression of sardonic humour; huge, smudged eyes. It’s the sort of photograph you would expect to emerge from a celebration - a wedding, perhaps, or a night at the opera - and it has probably been published by just about every newspaper in these islands. But it would never have been published at all, had not the bloodstained body of 19-year-old Patricia Curran been found near her home at Whiteabbey, on the outskirts of Belfast, in the early hours of November 13th 1952.

“To be honest”, says Eoin McNamee, whose new novel, The Blue Tango, is based on the murder investigation, “what really got me interested in the case was that photograph. It was reproduced a lot in newspaper articles, but the odd thing was that when we actually went to find out where it came from, nobody knew who had taken it. I think it’s possibly a clip from a yearbook or something like that; it looks like a deb’s photograph.”

The mystery deepened. But photographs, especially still photographs, have a kind of eerie life of their own. And as Patricia Curran’s photograph was reproduced again and again, its good girl/bad girl ambiguity combined with hints and suggestions that the morals of this good Protestant girl had been somewhat on the loose side, to produce an impression of a gaunt, sex-haunted femme fatale.

“If my book is about any one thing”, says McNamee, “it’s actually about Patricia Curran and who was Patricia Curran. Which means the book is also about women and images of women - mothers and daughters, and that whole ‘woman as victim’ thing which hangs over certain sex crimes, that she must have somehow deserved it.”

An early report carried by The Irish Times certainly does nothing to dispel that impression. Written in the terse, restrained style of the day, the story looks deceptively straightforward, its elements all neatly in place: the respectable family (”daughter of Mr Justice Curran, a judge of the Northern Ireland High Court”), the big house (”the drive up to the house is nearly a quarter of a mile long”), the unseemly scuffle, with its distinctly sexual undertones (“her yellow beret and high-heeled shoes were found some distance away ... the body had apparently been dragged ... to the bushes ...”). Patricia’s death, however, was anything but straightforward - and as it turned out, she was not to be the only victim of the frenzied attack in which, according to post mortem reports, she was stabbed 37 times.

By the middle of January, the murder investigation had made little progress. Then the detectives in charge of the case arrested a young Scottish serviceman, Iain Hay Gordon, who had recently been posted to the North by a sergeant who reckoned his marching wasn’t up to scratch. A shy, nervous loner, Hay Gordon had met Patricia’s brother Desmond at church and been invited to dinner at the Curran house, where he had met Patricia. As suspects go, he was a poor one: but he was the only one - and he had no alibi. Within days of his arrest, Hay Gordon had signed a confession. He was convicted. He might well have been hanged, but his defence changed his plea to guilty but insane and he spent seven years in a mental hospital in Antrim.

The neatness of the case, however, was never more than superficial, and from the beginning it had begun to unravel. Iain Hay Gordon had always claimed that his “confession” was fabricated, and that he signed it only when detectives threatened to tell his mother he was a homosexual. There had never been any forensic or witness evidence against him. Besides, there were just too many unanswered questions about the night of the murder. Why were Patricia’s family allowed to move her body from the crime scene and drive it to the surgery of the local doctor, claiming she was still alive despite obvious signs of rigor mortis? Why - given that she had been stabbed 37 times - was there so little blood? Why were her belongings, found yards from her body, not noticed at all until the following day - neatly placed at the edge of the driveway, and dry, after a night of driving rain? Why was the Curran house not searched for a week after her death?

McNamee’s book cleverly exploits the shifting sands of truth and falsehood. It is an imaginative triumph, a masterful recreation of both the period and the multi-layered tragedy of Patricia Curran’s life and death. It reads like a thriller, but it invests the bleak ordinariness of life in 1950s Northern Ireland with occasional jewels of poetic detail: “the hiss of sleet on water”; “car tyres going past in the rain, a lustful and godless whispering”; “glass doors bore faded gilt signs for loss adjusters, shipping agents ... repositories for the lore of empire”. Like the dance of the title, the book moves gracefully around both murder and miscarriage of justice, illuminating in the process a stratum of Northern society which fiction has found it more comfortable to ignore.

”There’s that rat under the sideboard thing, isn’t there?” is McNamee’s cryptic comment on the murky domestic politics of polite Protestantism. “It’s an interesting aspect of the North - and it still exists, that elite consensus, though it’s very archaic now. “They want it to be sort of Home Counties, but even the Home Counties aren’t like that any more. Postcolonial is a kind of hackneyed phrase, but that’s what it’s like - it’s like going to Cyprus and finding that they still drive Bristol buses.”

Before it is anything else, he says, The Blue Tango is a novel. So why did he use the main characters’ real names, mixing them with people of his own invention? “What actually happened”, he says, “was that I wrote the book using all the real names, with the intention of going back and changing them when I got to the end. I mean, there wasn’t any deliberate intention of setting out to create a new genre or anything. And then when I got to the end and thought about changing the names, I asked myself, ’why am I doing this?’ It seemed like an almost unbearably coy thing to be doing - I mean, the case is there, the book is about the case, so why change it?” Legal reasons is one answer that springs to mind, but McNamee shakes his head.

”Publishers being publishers, the lawyers had a look at it, and they seemed satisfied. I don’t think there’s anything defamatory of anyone in there. It’s more a question of how much are you entitled to recreate of people’s characters and their daily doings.

”Newspapers do it every day. It’s the business of historians to recreate the past through whatever glass they want to see it through. Is it the business of fiction, though? I’m not quite certain. It asks more questions than it answers - even to me.” McNamee says that if the book has a literary model, it’s Truman Capote’s In Cold Blood. “I’m glad I didn’t re-read it until after I’d done this, because it would have been very easy to pick up on the style.” But he doesn’t see The Blue Tango as an angry or a campaigning book. “The whole thing leaves a melancholy impression - it’s kind of hard to get angry when the perpetrators of whatever wrongs were done to Patricia and to Iain Hay Gordon are long gone.

”But perhaps what attracted me to the story is the very fact that it’s outside the literary pages, less abstract, more attached to life in a way. All real stories have structures which you couldn’t create in fiction; but they all have an internal dynamic which carries them, and maybe that’s what the attraction is, to try and find that dynamic.”

The campaigning was done by others, starting with Iain Hay Gordon’s elderly parents. Having received no treatment at Holywell mental hospital - in spite of the “guilty but insane” verdict - he was released into their custody and allowed to work in Glasgow on condition that he changed his name to “John” and never talked about the case. When he took voluntary redundancy in 1993, he began a long fight to clear his name; with the help of lawyers, politicians and a journalist called John Linklater - who is writing his own book on the case - he finally succeeded in having the guilty verdict overturned.

The mystery, however, remains. Who did kill Patricia Curran? Like Linklater, McNamee has his own theory, which is spelled out pretty unambiguously in The Blue Tango - and which, without giving too much away, keeps it in the family. Patricia’s brother Desmond - now a Catholic priest who works in a South African township - may be the only person alive who knows what really happened that night in November 1952, but he continues to insist that “no member of the Curran family was in any way involved in the murder or any kind of cover-up”.

One way or another, this tale of large men in suits rallying round to protect their own is unlikely to rehabilitate McNamee in the eyes of the British establishment. He ran the gauntlet recently for his work on the feature film of his last novel, Resurrection Man, on the subject of the Shankill Butchers. “The Tory press in England lost their heads over it - ‘a poisonous outpouring of anti-Unionist bile by Irish writer Eoin McNamee’ - and it was effectively censored out of existence.” The Blue Tango on the Booker shortlist? Place your bets now, folks ...