Juno and the Paycock is set in the Dublin tenements, or slums, during the Irish Civil War. It tells the story of the Boyle family: Jack Boyle, a self-centered man who spends his time drinking with his pal Joxer rather than looking for work; his wife Juno, responsible and hard-working, who takes care of the family; their son Johnny, who lost an arm in the Easter Rising and spends his days fearfully at home; and their daughter Mary, a young and somewhat vain idealist currently on strike.
The family learns from Mary's fiance Charlie Bentham that they will inherit money from a relative of Boyle's who has died. Upon hearing the news, Boyle borrows money from anyone who will lend it to him and makes purchases on credit, including furniture, a suit, and a gramophone. The family, along with their neighbor Mrs. Madigan, celebrates with an evening of song, although their spirits are dampened by the funeral procession of their neighbor Mrs. Tancred's murdered son.
Tragedy strikes when Boyle finds out that Bentham, who drafted the will, did so in such a way that all of Boyles' first and second cousins are eligible to receive a portion of the property, thus rendering the legacy worthless. Bentham breaks his engagement to Mary and retreats to England. Mary finds out she is pregnant and is consequently shunned by her male relatives and former suitor. As furniture men are emptying the apartment of furnishings, two Republican soldiers drag Johnny away; it turns out that he had given the information leading to the murder of Mrs. Tancred's son, and he is consequently murdered as well.
Juno decides that she and Mary will live with her sister and raise the baby together, leaving Boyle to fend for himself. The play ends with Boyle and Joxer returning to the stage drunk, Boyle retreating into a world of alcohol-induced fantasy rather than facing his problems.