Sebastian Barry’s The Secret Scripture, about the journal of a mental patient nearing her 100th birthday, was ­on Tuesday named Costa Book of the Year.

The novel had been shortlisted but ultimately overlooked for last year’s Man Booker prize.

Matthew Parris, chairman of the Costa judges, said the decision to award the prize to the Dublin-born novelist had been taken after an “extraordinarily close finish” against the other main contender, The Broken Word, Adam Foulds’s poetry sequence dealing with the Mau Mau uprisings in Kenya.

“There was a huge amount of support for both,” he said at the award ceremony in London. “It was a five-against-four decision which was on a knife-edge right up to the end.”

Mr Parris said the novel, published by Faber and Faber, had won the judges’ support in spite of the fact that “there was a lot wrong with it – it was flawed in many ways”.

The book, which was 2-1 favourite to win the prize, contrasts the journal kept by the patient, Roseanne McNulty, with that of her psychiatrist Dr Grene, to present an alternative history of Ireland. The novel was strongly acclaimed by critics and only just lost out to Aravind Adiga’s The White Tiger in the Man Booker prize.

Mr Parris said almost none of the judges liked the novel’s ending and the voice of Dr Grene had not worked as well as that of Roseanne.

“But we felt that, in Roseanne, a narrative had been created of such transcendence that it redressed all the other weaknesses in the book. A lot of us thought it was in its way a poem.”

Mr Parris said The Broken Word had also been strongly championed by some of the judges. “It is very narrative poetry – not everybody will see it as poetry, but everybody will see it as powerful and beautifully written.” Mr Parris urged readers to buy both books, adding that he was encouraged that all the judges had been concerned with accessibility.

Mr Barry will receive £25,000 in addition to the £5,000 he won as winner of the contest’s best novel category. His previous awards include the Christopher Ewart-Biggs Prize, the London Critics Circle Award and the Kerry Group Irish Fiction Prize. The other shortlisted titles for Book of the Year were The Outcast by Sadie Jones; Somewhere Towards the End by Diana Athill; and Just Henry by Michelle Magorian.