The key theme in The Cask of Amontillado is irony and this is shown throughout the story in many ways. The story is narrated from the point of view of Montresor who has sworn revenge on Fortunato. The mood of the story and way it is told, through the mind on Montresor, sets an evil premise, but irony is the key focus of the story.
Irony occurs in several ways. The most noticeable one is that the "insult" that motivates Montresor's revenge does not warrant murder. The reader never learns specifically what Fortunato has done but it is clear that Montresor has gone insane, and Fortunato is innocent of any crime. This is ironic for a very obvious reason. The whole basis of the story is revenge, but it is not justified. Montresor has no honor to defend but he still feels justice is no being served as long as Fortunato is alive. A second example of irony is that Fortunato is in a life or death fight with Montresor but never realizes it. He believes he is friends with Montresor and is in safe company as he is lead to his death. We believe that if a man is being hunted due to an insult of another he would have an idea about it. Fortunato, however, does not know until he is chained and buried alive. It is also ironic that Montresor gets away with the murder. There is a sense that he and not Fortunato will, in the end, be punished. We are led this way up until the climax, when Fortunato is chained and we realize that Montresor's revenge will be carried out. The final piece of irony is that justice being served is a key point of the story but only injustices happen. Montresor is the wrongdoer who deserves to be punished and Fortunato is no more then an innocent bystander. The complete opposite of what rightfully should happen is what truly happens. Montresor tells the story years later without ever even being suspected of the crime, while Fortunato dies a horrible and lonely death. .
Irony presents itself in many ways in The Cask of Amontillado.