Joyce's claim
Joyce's claim that she witnessed a terrible murder is met with scorn and derision. She tells everyone about the murder to bring attention to the heinous act. However, no one believes her, ironically leading to her being murdered by the same person.
Mrs. Drake
Mrs. Drake, the woman who threw the party Joyce and so many others were at, prioritized the state of the apples in her apple-bobbing game over Joyce's death. Usually, people value human life over anything else; Mrs. Drake ironically values apples, which have little worth, over a human being, which has a tremendous amount of worth.
The garden
Gardens are a place of life, renewal, and beauty. However, a particularly brutal murder is unexpectedly committed in a garden. This irony further underscores the depravity of the murderer.
Rowena Drake
The novel's murderer that Poirot seeks out turns out to be Rowena Drake. Drake is a respected member of her community and the organizer of the very Halloween party where Joyce is killed. No one around her expects her to be the culprit because of her demeanor, personality, and kindness. But she is a cold-blooded murderer.
