Poor Skippy. The title tells you what happens to him, and the prologue tells you when and where: Daniel “Skippy” Juster keels over in the middle of a doughnut-eating race against his corpulent maths-genius room mate, Ruprecht Van Doren. But it takes you the rest of the book to find out how and why - and what Skippy’s death means to the people around him.

Paul Murray, author of the Whitbread-nominated An Evening of Long Goodbyes  (2003), returns after a seven-year hiatus with a bang: Skippy Dies  is a blast of a book. It’s big, generous, heartfelt, funny and sad. If it sometimes seems in danger of tipping into cartoonish sentimentality, then it just as often redeems itself with a sharply phrased perception here, a throwaway gag there. It seems destined to be a cult novel, the sort of book you want to press on people so you can talk to them about it - what did you think of the bit when ...?

Skippy (so-called because his buck teeth make him sound like Skippy the Bush Kangaroo) is a second-year boarder at Seabrook College for Boys, a Dublin institution reminiscent in several of its particulars of the immortal edifice that commands Rock Road, Blackrock College. While Blackrock is run by the Jesuits, Seabrook is run by the Holy Paraclete Fathers, a moribund order of missionaries who are in danger of being edged out by the advance guard of the private-school laity, represented by the unctuously fascistic acting principal, Greg “the Automator” Costigan - one of the many vividly conjured grotesques that people the pages of Skippy Dies. (The Automator gets some great lines; for example: “She’s an investment banker, she’s not used to that kind of unbridled depravity.”)

Revolving around Skippy is a cast of eccentrics, perverts, sociopaths, confreres, teachers, bullies and (most importantly and mysteriously in the eyes of a teenage boy) girls. The novel summons up the cosy, ritualistic atmosphere of the traditional British boarding-school mock saga - there are scrapes and adventures, such as a night raid on St Brigid’s, the girls’ school next door, in search of Ruprecht’s interdimensional transport pod - but Murray isn’t interested in limiting himself to writing a boarding-school comedy. There are several narratives woven through Skippy Dies , and what happens to Skippy himself is only one of them.

There is Skippy’s history teacher, Howard Fallon, known as Howard the Coward, and his passion for Aurelie McIntyre, the enigmatic substitute geography teacher ("Not-to-be-taken-Aurelie,” chuckles Howard’s friend Farley); there are Carl and Barry, the school psychos, whose descent from prank-playing schoolkids to drug-dealing monsters is rendered in a manner that impressively avoids cliche; there is Skippy’s French teacher, Father Green, whose name in French - Père Vert - isn’t simply a coincidence; there is Tom Roche, coach of Skippy’s swimming team, whose career-ending injury has something to do with where Howard the Coward got his nickname; there is Lori, object of desire for both Skippy and Carl (and how difficult is it to write about a teenage girl without resorting to cliche? Yet Murray manages it; Lori is both innocent and corrupt, when most writers would have settled for making her one or the other); and, most centrally and entertainingly, there are Skippy’s classmates, including Ruprecht, the book’s best character, whose obsession with theoretical physics proves a deep thematic and comic resource.

The warmest and funniest moments in the book are the ones that feature the second-year boys of Seabrook. Murray has an astoundingly good ear for the way adolescent boys speak to one another - that affectionate raillery that occasionally shades into outright hostility ("I’ll saw my hands off before I appear on stage with you and your Orchestra of Gays!”) - and an uncanny understanding of the way teenagers think and feel. The comic exaggeration of the Halloween Dance sequence, in which a gymful of 14-year-olds vomits en masse because of spiked punch, is underwritten by the light touch with which Murray lets us see the sexual anxieties of these boys and girls: “God, I’m so sick of these fucking boys,” a St Brigid’s girl declares. “I need a man”; and Mario, Skippy’s Italian classmate, describes the girls as “skittles, waiting to be bowled over by Mario’s big balls”.

But the entire book is full of beautifully observed moments like these. The Automator, in particular, is a pitch-perfect parody of a kind of blindly loyal, jargon-spouting ideologue with whom we’re all familiar. Of the fishtank in his predecessor’s office, the Automator remarks: “Fish aren’t team players. Look at them. There’s no system at work there. They’re not even talking to each other. How are they going to get anything done, you may ask? Answer: they’re not.”

There’s a lot going on in Skippy Dies. It’s an ambitious book, and it aims to encompass a great deal in its tragicomic sweep: from Howard’s obsession with Robert Graves and the first World War to Ruprecht’s fascination with M-theory (a possible explanation of the origins and nature of the universe) and interdimensional travel, Paul Murray gets a lot in. But he never loses sight of his characters and their world: by the time Skippy’s death arrives you’re valiantly hoping the title is a lie, or another joke. Alas, it is not. Skippy Dies  is impressive in all sorts of ways, but none more so than in its ability to move you. Let’s hope we don’t have to wait seven years for the sequel.