Patrick McCabe, like his fellow Patrick - the splendidly dour McGrath - is haunted by insanity. His last novel, 1992's The Butcher Boy, told the sorry tale of Francie Brady, homicidal progeny of an alcoholic and suicidal household. Much the same attends these pages, albeit on a bigger canvass.

Essentially this is the story of two men and how they almost innocently but effectively destroy one another. Raphael Bell, who had a father murdered by the Black and Tans, was a precious, precocious child grown to diligent, pious adulthood, decently married and proud headmaster - "king of all headmasters" - of the prestigious St. Anthony's. Malachy Dudgeon's childhood was, if anything, even more traumatic - driven apart from his beloved mother when he inadvertently witnesses her Sunday afternoon infidelities with the cowman; haunted by his father's subsequent suicide - but he survived, and his career at teacher training college was successful if unremarkable. If nothing else he met and fell for the strawberry-blonde Marion.

It's when Malachy comes to St. Anthony's that fate deals both men the mortal hand. Conflict between Malachy's more casual, contemporary ways (the book's set largely in the 70s) and Bell's rigid, almost fascistic traditionalism is inevitable and swiftly out of bounds. Bell is convinced the new man is undermining his authority, while each night Malachy is wracked with dread at the inability to master his young charges. Tragedy eventually destroys the careful life each man has built.

McCabe is not altogether able to surrender to his psychosis sufficiently to pen a book quite as extraordinary as McGrath's Spider, but working on more or less similar ground to his last he fashions a much better, much more driven, much more readable novel. The style, like that book however, is initially problematic. McCabe makes no concessions - early chapters are dense, sentences stretch on line after line after line. It demands commitment from its readers but rewards them with idiosyncrasy and humour. Witnessing with horror the descent of Ireland into promiscuity and lax morality, and his beloved IRA turning to violence, Raphael Bell becomes apoplectic at the very mention of bras on the radio.

There is more than a touch of the Grand Guignol in all this. Still births, abortion, Bell's dreams of his dead son, overdose, suicide, drug dementia. All this and 'Chirpy Chirpy Cheep Cheep'. McCabe instinctively understands this stuff and makes effective use of it, even if the authorial voice he assumes - life is disappointing, "boys and girls" - is occasionally patronising. The book is best when it submerges the narrator in his story.

The central strength of the novel is also its main failing. 350 pages is a good fifty too many, but is a size dictated by the very style of the thing. McCabe has to follow these people through to the bitter end of their shared mania, but in doing so is forced to open-out the narrative too far. He risks the whole thing collapsing in on itself. It doesn't, but cracks do show.

At its best - the central third, when both men clash at the school - the book is superb. It has a subtlety and wit that towers over The Butcher Boy. Its entry into the world of pettiness and betrayal is remarkable, its celebration of McCabe's interest in monstrous children acute. As his last was a good book, this is a very good one. Eventually he will write a great one, but for the moment The Dead School is not quite it.