Northern Ireland, 1961. The body of a young woman, stripped naked, brutally beaten, stabbed and finally strangled, is discovered in a stubble field after a dance at Newry Orange Hall. Though the police have nothing to go on other than the most circumstantial evidence, the whole town agrees that the killer is a young bodybuilder and ne’er-do-well named Robert McGladdery. In spite of the efforts of Inspector Eddie McCrink, a native of the area newly returned from a failed serial-murder inquiry in London, everyone is determined that the killer must hang. McCrink, the one honest officer on the case, has serious doubts about the witness evidence and about the motives of his colleagues and superiors, but it doesn’t really matter what he does; as one local says to him: “You’ll never find out what happened that night ... It’s the town’s business and the town will take care of it.”

Taking care of it, in this case, means many things, from official meddling to dubious testimony and low-level police brutality, and McCrink’s fundamental mistake is to imagine that hard evidence and reasoned thinking will have any influence on the final outcome. Meanwhile, the accused man’s case is not helped by the fact that Judge Lance Curran, whose daughter Patricia was murdered several years earlier in almost identical circumstances, is on the bench.

Such a summary makes Orchid Blue sound like a common or garden crime thriller, but anyone familiar with McNamee’s 2001 Booker-longlisted Blue Tango – in which he conducted a fictional investigation into the real-life murder of the same Patricia Curran – will know that there is more to this narrative than meets the eye. The case at the centre of Orchid Blue is also drawn from the history books and, once again, McNamee reveals how unsound the legal process can be when it comes to investigating high-profile murders. McNamee’s method of combining historical research and speculation ought to put us on our guard (as some critics noted of Blue Tango, it is hard to know where reportage ends and invention begins); what happens, however, is quite the opposite. Like its predecessor, Orchid Blue makes us ask about what we can know and what we can rely on, especially when the authorities are involved – and, because everyone is implicated in the lethal flow of misinformation, political scheming and superstitious thinking, the book can scarcely avoid presenting us with a wonderfully complete study of the social order, from the arrogant Minister for Home Affairs (“Faulkner was a big-house Unionist. He had estates in the east and on the border. An air of feudal rancours about him. Disdain in the gene code.”), down through the sin-haunted judge and his truly repulsive fixer, to the compromised police officers who do their masters’ bidding and the poor mill workers and travelling folk kept in their place by religion and bigotry.

The economy of McNamee’s prose, the way he can use a minor detail or a glib turn of phrase to move us to pity or righteous anger, is equally impressive. He does so much with “the word Ladybird” on a victim’s slip, or the passing description of the flick knife Robert buys with money stolen from his mother’s purse, while his portrayal of vile officialdom – the corrupt but untouchable political fixers; the compromised policeman, in the bosses’ pockets after a sexual indiscretion; the thuggish sergeant who enjoys the casual brutality of his work – is powerfully damning while never straying into sensationalism.

Meanwhile, an atmosphere of dark, damp menace runs through the novel like mildew, an underlying sense of flawed blood and brute superstition that informs every witness statement and every unsound judgment. Robert’s guilt is decided as much by the taint of the places he frequents as by the circumstantial evidence produced in court: the narrative that people construct from tribal memories and fears is, for them, far more decisive than an alibi or a fingerprint. “As in much of the evidence there are other subtle undertones in the policemen’s statement which the prosecution would have been aware of. The fact that Robert had gone to a “semi-derelict house”. The semi-derelict house on the edge of town something which had worked its way into the popular imagination. The dimlit interiors, the rubbish-strewn rooms and the faded wallpapers. People felt that some old magic was at work in them, a watchfulness. Places that were home to half-remembered happenings, folkloric terrors.” That the authorities are quite capable of using those “folkloric terrors” to their own advantage – the minister needs to seem hard on crime, the judge is looking for advancement – comes as no surprise.

What ought to come as a surprise, however, is the way that ordinary people - witnesses, suspects, victims - betray themselves, almost casually playing into the hands of powers that they should be questioning and opposing. These people - the lower classes, the neglected - are, in fact, so desperate to become part of the story that they will accept any role they are offered: “Hesitant at first. Then a story starts to take shape in their heads. They start to see the possibilities of narrative, the interwoven stuff of their lives. How things have shaped themselves around the defining moment ... They try to piece the night together, how seemingly meaningless events become part of the weft. They start to ponder the interrelatedness of things. They are grateful to the interviewer for helping to find the patterns, leading them towards the ornamented telling.”

It is this sense of how the defining moments come to be agreed - of how they are essentially defined by the ruling class - that illuminates Orchid Blue, so that what begins as a crime thriller gradually builds not only into a political novel of the highest order but also that rare phenomenon, a genuinely tragic work of art.