Bog Child, the story of a teenage boy who finds the body of a child in an Irish bog, was finished by Dowd in May 2007. She died of cancer that August at the age of 47, having only turned to writing in 2003. In just four short years, she penned four children's books: her first, A Swift Pure Cry, was also shortlisted for the Carnegie.

"It's infuriating that she didn't start writing earlier, that she couldn't go on. We've lost one of our great new voices, and they don't come along that often, not at Siobhan's standards," said her publisher and editor, David Fickling, who accepted the Carnegie medal on her behalf this lunchtime. "Bog Child was written with great intensity, when Siobhan was at the height of her powers, all the while being very ill ... You get to the end and are uplifted, and that's what she was like in person, too. She buoyed you up."

The book is "an absolutely astonishing piece of writing", said the librarian Joy Court, chair of the judging panel (the Carnegie medal winner is selected by 13 librarians from around the UK). "To be able to write like that when she was going through what she was going through is just astonishing – the sheer beauty of the language, the descriptions of the environment; she has such an amazing sense of place."

Bog Child intertwines two stories: that of the 16-year-old Fergus, who discovers the child in the bog in 1981 and thinks she has been murdered by the IRA, and that of the bog child, Mel, who turns out to have lived 2,000 years ago during the iron age. Fergus smuggles packages across the Northern Ireland border each day, believing them to contain semtex, while his brother goes on hunger strike in prison in an attempt to free Northern Ireland from "the misery of it. The mourning and the weeping. The vale of tears." Dowd's command of language is "extraordinary", said Fickling, as in her description of Mel's death: "Silver light fizzed and shot apart. Love fell in particles, like snow."