The Gamal, By Ciarán Collins. Bloomsbury Circus, £12.99

"Once upon a time," begins Ciarán Collins' debut novel, "there were two lovers called Sinéad and James." She is a local girl from a poor Catholic family where her home life is troubled and her parents abusive, while James is the son of a well-off Protestant couple who move to the area to renovate an old castle.

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From the moment they first meet in primary school, Sinéad and James are destined to be together, but, as in all tales of star-crossed lovers, love also breeds jealousy and hate, and fairy-tale can turn to tragedy. Scratch the surface of the small town where they live and sectarian grievances are still bubbling away. Not everyone believes their story should end happily ever after.

Our narrator is 25-five year-old Charlie, the "gam" or "gamal" (taken from the old Irish word Gamalóg to mean a simpleton or fool): Sinéad and James's classmate, friend and confidant. When the novel opens we know there's been a death or deaths, a court trial, and that Charlie is suffering from post-traumatic stress disorder, but we don't know the facts of the case. His psychiatrist has told him to write his story, but Charlie's not doing it for therapy; he's hoping to make enough money to get out of Ballyronan, the (fictional) Irish village where the action unfolds.

Because he's the gamal, people don't pay much attention to him. But he pays attention to them, noticing the "small things", and so should we. Charlie, however, is both a reticent – his is a book for people who, like him, "hate reading"; he draws pictures rather than describe things – and an unreliable narrator. He presents the facts in the "wrong order", and keeps certain secrets. In one way or other, Charlie's story is a confession, but what exactly he's guilty of is left up to the reader to decide. Reading The Gamal is indeed part-jigsaw puzzle, a process of piecing the various clues together, but it's a richer experience for it.

Charlie holds his own against any precocious child narrator, and Collins's brave decision to end his novel with questions left unanswered is brilliantly confident. Genuinely heartbreaking in parts, The Gamal is a gritty, modern Romeo and Juliet told by a compelling and original voice.