As the title indicates, The Beauty Queen of Leenane is set in Leenane, in Western Ireland. The play centers around Maureen, an unmarried woman in her early 40s, as she cares for her ailing mother, Mag. The two share a turbulent and unhealthy relationship. Mag taunts and insults her daughter, and Maureen blames her mother for her unhappiness. The play begins as the two women are invited to a going away party hosted by a neighbor. The two begin to fight, and Maureen decides to attend the party alone.
At the party, Maureen meets Pato. While the two have known each other for many years, they never spoke to one another. They bond over their previous experiences of working in England, and Maureen brings Pato home for a sexual encounter. The following morning, Mag sees Pato in the kitchen. She and Maureen begin fighting again. In the fury of the interaction, Mag reveals that she is actually Maureen's caretaker, not the other way around. She goes on to say that Maureen was committed to a mental institution, and that she is still mentally unwell. Maureen retorts by saying that her mother is senile and cannot discern fact from fiction. The viewer is left to question who is truly sane and who is the one telling the truth.
After the fight, Maureen reveals to Pato that she suffered a mental breakdown many years prior. He remains supportive, though she begins to grow upset. He leaves hastily but promises to keep in contact. In a later letter, Pato says that he will be moving to America and that he wants Maureen to join him. Before Maureen can read it, Mag intercepts the letter and destroys it. In a later conversation Mag mistakenly mentions a secret shared between Pato and Maureen, thus revealing that she intercepted the private letter. Maureen is outraged and begins to batter her mother. She then proceeds to douse Mag in hot oil, until the old woman confesses to reading the letter. Maureen then leaves the house to attend Pato's going away party.
When Maureen returns from the party, she says that Pato and her agreed to remain faithful to one another. Maureen then bashes Mag's head in, killing her. The scene then skips to Mag's funeral, where somehow Maureen has not been arrested for Mag's murder. Pato's brother is at the funeral, and it becomes evident that Maureen never saw Pato at his going away party. After all, it becomes clear that Maureen is the insane character. The play ends as she rocks in Mag's chair while wearing one of her sweaters.