‘By Christ if you were mine I’d break every bone in your body,’ says the police sergeant to Francie Brady, the butcher boy narrator of Patrick McCabe’s splendid third novel. ‘Not that you could be any different.’

When we first meet Francie, he is hiding under a hole in a hedge while the inhabitants of his small County Monaghan town scour the country for him ‘on account of what I done on Mrs Nugent’.

Precisely what Francie done on Mrs Nugent will be revealed eventually, in the course of his grim and often blackly funny story. The auguries are not, from the beginning, good. The Brady family might reasonably be described as dysfunctional, and are known locally as ‘the pigs’. The Da and his brother, Uncle Alo, grew up in a Belfast orphanage waiting disconsolately for a father who never came, and now the Da immerses himself in drink and lives on tins of pilchards. Uncle Alo, the family success story who got away to Camden Town and did well for himself, is a factory security man, ‘tipping his cap to his betters in his wee blue porter’s suit’.

A far cry, then, from the Nugents. Philip Nugent takes piano lessons, has an enviable collection of comics and wears a smart school blazer. ‘He was like Winker Watson out of the Dandy in this get-up of his only Winker was always up to devilment and Philip was the opposite.’ At the start, Francie is more of a real-life, flesh-and-blood Bash Street Kid, dumped fully formed in John McGahern country, and he stays in the saucy-grim world of DC Thompson while his friends move on to higher things. ‘Dan-ta dan!’ he announces himself, setting off, like Patrick Kavanagh, to walk the 50 miles from Monaghan to Dublin, sleeping under bushes ‘and once in a tyre’. It is this flight that provokes the suicide of his mother, but Francie blames the Nugents, and takes his cursory revenge by desecrating their neat and polished home. What do pigs do? ‘They do poo! Yes, pigs are forever doing poo all over the farmyard, they have the poor farmer’s heart broken.’

Which is how Francie ends up, like his father, in a church-run home for wayward boys. And wayward priests, for that matter, whom Francie placates by witnessing apparitions of the Virgin Mary. ‘It was like all the softest women in the world mixed up in a huge big baking bowl and there you have Our Lady at the end of it.’ If only Francie can win his yearned-for ‘Francie Brady Not a Bad Bastard Anymore Diploma,’ then perhaps things will turn out for the best.

Perhaps, indeed. Francie is fortunate enough to land a job at the local slaughterhouse where he gains further insight into the fate that the world reserves for pigs: ‘He was as pink as a baby’s bottom and he said to me with his big eyes: I’m not a big pig yet I don’t understand anything. Please - will you not let any harm come to me?’ and a few seconds later, ‘down on the concrete plop and not a squeak out of him all you could see was him saying you said you’d mind me and you didn’t’

And no one has taken the trouble to mind Francie, either. The account of his vertiginous slide into delinquency, madness and much worse is masterfully achieved. As his story lurches towards the inevitable horrors of its conclusion, the schoolboy chuckles turn slowly, almost unnoticeably, into chilling snorts of genuine insanity. To overstress the compassion with which McCabe treats his protagonist might risk making this powerful book, with its fierce, black humour, sound like a pious tract, and it’s anything but - it’s impossible not to be drawn into Francie Brady’s world, but Christ knows you wouldn’t want him in your house. Yet the compassion is there. Like John Banville in The Book of Evidence, Patrick McCabe has given us a shockingly intimate portrait of a mind out of kilter, but in the company of Francie Brady he guides us a good few steps further into hell.