Situational irony refers to events in a story that are unexpected, and Poe's "The Cask of Amontillado" features many, including the difference between the meaning of Fortunato's name and his destiny, as well as Montresor's response to his own deeds. The fact that the reader is well aware of the growing irony gives the story a great deal of its power.
The very name "Fortunato" teems with irony, because despite his name meaning "lucky" or "fortunate" in Italian, he ends up imprisoned behind the walls of the catacombs. Once Montresor decides to undertake revenge against Fortunato, he "neither by word nor deed" gives Fortunato a reason to doubt his friendship. The whole time that Montresor continues to smile at Fortunato, though, he is plotting Fortunato's doom. When Montresor greets Fortunato, saying, "You are luckily met," the luck that Montresor feels is not due to Fortunato's expertise. Rather, it is that the carnival provides an excellent backdrop for a secret murder.
Finally, after carrying out the crime, Montresor describes his heart as growing sick, but it is because of the "dampness of the catacombs." Rather than fret about the murder he has just committed, he is worried about what is going to happen to the weather outside.