The Country Girls: Three Novels and an Epilogue explores what it means to be poor, female, and Irish. The novel follows Kate and Baba, who represent a demographic seldom explored in literature: Irish women. As dual protagonists, they are, in some sense, foils of each other; Kate is shy, while Baba is assertive. In another sense, they are quite similar; both seek love in the wrong places and are compelled to conduct unfulfilling extramarital affairs.

Kate is victimized by circumstance after her mother drowns and she is left with her father, an abusive alcoholic. Baba's father is kind, but Baba is unintelligent. They become friends when they are expelled from a convent school and live in Dublin together. O'Brien's set of novels explores what the city has to offer the uninitiated countryfolk. Kate works in a grocery store and Baba goes to school; however, Kate falls in with a married man who tries to seduce her. Eventually, Kate has a child with this man, but later has her own extramarital affair with a married man.

Baba (who narrates the third novel), too, gets married, but becomes pregnant with a child as the result of an affair with a drummer. Her husband accepts the child, but does not like Kate, and so the two lose touch until a week before Kate's death.

The novel explores the circumstance that women are often at the mercy of the men they choose and that patterns of human behavior (such as infidelity and alcoholism) are bound to repeat themselves.