Flaubert once wrote that novels are the private histories of nations. Sebastian Barry’s subject is the history of his native land, Ireland, in the early part of the 20th century, with its shifting allegiances, its barbarous tribalisms and its long-remembered slights.

In Mr. Barry’s new novel that history is symbolized by a secret. And it is revealed to the reader as if a thread were being slowly unraveled from the cocoon of a silkworm to expose at its core a terrible truth.

In the modern-day West of Ireland, Roseanne McNulty, nearly 100 years old, has been imprisoned for decades in the Roscommon Regional Mental Hospital. This big 18th-century pile is about to be torn down, and the hospital’s chief psychiatrist, Dr. Grene, has come to evaluate Roseanne to see if she is fit to be released into the general population.

The two begin writing parallel accounts of their meetings, each of which becomes a secret scripture of their lives. Roseanne tells the story of how she, once a great beauty, came to be put in the home. Dr. Grene describes his own private anguish, the break-up of his marriage over his single infidelity, and his wife’s death, interspersed with his notes on Roseanne’s case.

At first his story pales next to Roseanne’s, with all its elements of passion, murder and betrayal, and there are times in the book when it seems doubtful that these two apparently distinct narratives can ever resolve themselves into a whole novel. But not to worry.

Roseanne is a Presbyterian, the daughter of Joseph Clear, a “keeper of the graves” in a Catholic cemetery, who may or may not have been in the Royal Irish Constabulary. A group of Irish Republican Army irregulars, opposed to the 1922 treaty with Britain that partitioned Ireland because they believe it gave the British too much authority, stumble into the family hut carrying a dead comrade. Roseanne is sent to fetch the local priest, Father Gaunt, to administer the last rites. (Catholic Church officials in Ireland opposed the renegades.) Later, Roseanne is accused of betraying the boys to their former allies, the Free State Army, which favors the new treaty, and with whom the I.R.A. irregulars are conducting a bloody civil war of their own.

Roseanne’s father loses his job and becomes the village rat catcher. He is later found hanged, whether by murder of by suicide is not known.

Re-enter the aptly named Father Gaunt, who is a masterpiece of cold dominion if there ever was one, a hater of women’s sexuality, “cleaner than the daylight moon,” who tells Roseanne that he can’t have a beautiful girl like her running about as a temptation to all the young lads of Sligo. He arranges for her to marry a Free Stater, Tom McNulty (the brother of the title character of Mr. Barry’s novel “The Whereabouts of Eneas McNulty.”)

But the young bride is spotted talking to one of the I.R.A. irregulars, the brother of the dead boy. For this simple transgression she is banished, by her husband’s family and Father Gaunt, to a life of complete isolation in an iron hut at the edge of the sea. Father Gaunt arranges for her marriage to be annulled on the ground that she is a nymphomaniac, and he schemes to have her imprisoned in the mental institution. When crossed, Roseanne notes, Father Gaunt “was like a scything blade, the grass, the brambles and the stalks of human nature went down before him.”

Mr. Barry has said in interviews that many of his characters are based on his relatives. His great-grandfather James Dunne, the last Dublin metropolitan police superintendent before the transfer of power from the British in 1922, inspired the central character in his play “The Steward of Christendom.” The life of another ancestor provided the foundation for the main figure in his novel “A Long Long Way,” which was short-listed for the Man Booker Prize. And the character of Roseanne McNulty, Mr. Barry has said, is loosely modeled on a great-aunt, who similarly disappeared into a mental institution after a mysterious offense, possibly having to do with her being beautiful.

These lives are reimagined in language of surpassing beauty. A woman is as “young and slight as a watercolor, a mere gesture of bones and features.” Swans in a rainstorm are like “unsuccessful suicides.” And the moon — well the moon is “prince of all outside,” he writes. “Its light lay in a solemn glister on the windowpanes.” There is also in this book what has to be one of the most astonishing descriptions of a childbirth ever written.

Mr. Barry has said that his novels and plays often begin as poems (he is a published poet), but his language never clots the flow of his story; it never gives off a whiff of labor and strain. It is like a song, with all the pulse of the Irish language, a song sung liltingly and plaintively from the top of Ben Bulben into the airy night.