When is dramatic irony shown in the "Cask of Amontillado"?
The whole story is an example of dramatic irony because Fortunato is unaware of Montresor's plot to kill him while the reader knows this from the very beginning. Montresor goes through an elaborate plan to lure Fortunato to the catacombs beneath his house in order to punish him for an unknown insult Montresor feels Fortunato has inflicted upon him. We know what Montresor is doing, but poor Fortunato has no idea until it's too late. Poe's use of dramatic irony allows us, the readers, to be a part of Montresor's plan and to watch how he skillfully carries it out. We see Fortunato's reaction when he realizes what is happening. We know what's coming, but it adds suspense to see how Fortunato will react to it.
The entirety of The Cask of Amontillado is told with dramatic irony. Dramatic irony is when the audience knows something that a character in a story doesn't, it typically creates a feeling of suspense. In Poe's story Montressor reveals early on that he is going to get revenge on Fortunado. With this information we are forced to hold our breath as he plays out his grand scheme. Fortunado has no idea what's coming and that pushes the reader further into the story because they become invested in his eventual reaction.
Dramatic irony is when we as the audience (or reader) know something that the characters, or a character, does not.
As we read The Cask of Amontillado we know that Montressor is going to seek revenge on an unnamed insult on Fortunado. So as Montressor is luring Fortunado there is a heightened sense of conflict or suspense because we know the outcome is not going to be good for Fortunado.