What is ironic about the setting? In what ways does the setting suit the plot of the story in "The Cask Of Amontillado"?
The story (that Montresor tells) is set in Venice during the carnival season. Montresor calls it "the supreme madness of the carnival season." He means that the carnival celebration is at its height. Fortunato has been fully partaking in the celebrations and he is also dressed for the occasion. Montresor is not taking part. Montresor distances himself from others. Montresor has a French name. He shows his pride and mocks the Italians, saying that only few of them have the "virtuoso spirit."
The irony here is that while all others are dressed up, masquerading as different characters (Fortunato is dressed as a jester), it is Montresor who is also pretending to be someone he is not. He is friendly with Fortunato but only to lure him to his death.
With the story being set during a carnival (a sort of drunken Halloween celebration), it is ironic that the undisguised Montresor is actually playing a character himself. This suits the plot because Montresor's strategy is all about deception. He uses everything from reverse psychology to flattery to get Fortunato to follow him. It is ironic then that he (Montresor) is not wearing a disguise whereas Fortunato is wearing a costume because it is he who is deceived by Montresor.