In a mental hospital in modern-day Ireland, Roseanne McNulty sits; a taciturn old woman about whom little is known. She may be approaching her 100th birthday, but there is no family to celebrate or even confirm her age. Preparing herself for death, she resolves to write a “brittle and honest-minded history of myself”, stowing the loose, pen-scored pages under a floorboard in her room.

The hospital's senior psychiatrist, Dr Grene, is also in the habit of secretive scribbling, recording his impressions during the last days of his crumbling workplace, which is fated for demolition. As he assesses patients for release or relocation he finds himself drawn to Roseanne, and determined to tease out the truth of her internment.

The Secret Scripture alternates between the “secret scriptures” of Roseanne and Dr Grene - a curiously old-fashioned story-telling trick, which initially feels rather stiffly set out. But as Roseanne's pages turn, any hint of artificiality fades, and we are left with her lovely, chiming voice, relating her astonishing story with sadness and grace.

Roseanne grows up on the West Coast of Ireland, in Sligo; a cold, dark town assaulted by rain so brutal it makes the houses “shiver and huddle like people at a football match”. From the beginning she lives on the periphery: her beloved, kind father, Joe Clear, is a Presbytarian, respected but held at arm's length by the Catholic town.

Sligo itself has - along with the rest of the country - been splintered by civil war, sparked by the 1922 Anglo-Irish Treaty that partitioned Ireland into North and South.

Joe is proud of his job as “keeper of graves”, but fate will not leave him be. When a group of young Irregular (anti-treaty) fighters burst into the graveyard hut one evening, demanding that their dead comrade be buried, Joe and Roseanne are shocked - and as the consequences of the encounter play out, their family stability unravels.

Roseanne's mother drifts into insanity, her father falls from grace and dies. The priest, Father Gaunt, who wields enormous, insidious power, tries to take the teenage Roseanne under his wing.

At a Quaker-owned café she finds refuge, and eventually a husband, Tom McNulty - the charismatic leader of Sligo's only jazz band. But while their relationship blossoms, the ghosts of Roseanne's childhood - Father Gaunt included - are stalking in the shadows, ready to reclaim her.

Roseanne's eventual destination recalls the lunatic asylum of Barry's play The Steward of Christendom, a poetically meandering monologue from the former policeman Thomas Dunne, sectioned by his family and marginalised by the country he served.

Roseanne McNulty is not mad, but like Thomas Dunne - and like his soldier son Willie Dunne, whom Barry brought to life in his Booker-shortlisted 2005 novel A Long Long Way - she has been consigned to the margins of society and the footnotes of history.

In the telling, Roseanne's patchwork of memories becomes “history” - according to her own definition, a “fabulous arrangement of surmises and guesses held up as a banner against the assault of withering truth”. But it also becomes “scripture”: a sacred text. There is something spiritual in Roseanne's brave reverence for life, in her willingness to find angels in the midst of cruelty, prejudice and ignorance.

Dr Grene too discovers a belief in angels, despite his scientific bent - but his circuitous, erudite jottings cannot exert the same grip on the reader's imagination as Roseanne's blood-stained testament. As if to counter this imbalance, Barry gives Grene a dramatic revelation at the end of the novel, but it is too much, too neat, too late.

The plot might strain, but the prose holds fast. Roseanne's history is full of gleaming images - a soldier laughs “a horrible laugh like a lash of rain into your face”, the sand of Strandhill beach seems to have “drawn up its enormous knees” - and bright, “belling thoughts”. It's a story to treasure, and Roseanne is a teller to remember.

The Secret Scripture by Sebastian Barry
Faber, £16.99; 300pp Buy the book here

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