Montresor greets Fortunato at the carnival, and tells him that he is luckily met. Fortunato is not actually luckily met, because meeting Montresor that night brought him to his doom. Later in the story, he will be murdered by Montresor.
In this scene, Montresor presents Fortunato with some wine to sample. Then, he makes the comment that he is drinking to Fortunato's long life. This is an example of literary irony. It is ironic because Fortunato will not truly have a long life. In just a few minutes he will be murdered.
The reader knows that Montresor plans to seek revenge on Fortunato, and kill him, but Fortunato has no idea. When Montrsor meets Fortunato at the carnival, we know he plans to lure him away to kill him. Fortunato just thinks that he is going to taste some wine. Throughout the whole story we anticipate his murder, while he doesn't expect it. This is called situation irony.
In the story, Fortunato is wearing a fool costume. This is an example of symbolism, because his foo costume symbolizes who he is as a person. Fortunato is a fool, because someone he underestimates, and considers a friend is going to murder him. As the continue making their way through the catacombs, Fortunato never expects what is coming to him.
In the story, Poe mentions that it is carnival time. The carnival symbolizes Montresor's mind. Carnival's are full of craziness and madness, that is exactly how Montresor's mind is. He is plotting the murder of someone he calls a friend, and so I would consider him fairly crazy.
An example of foreshadowing was when Montresor pulled a trowel out of his coat. Later on in the story he uses this to trap Fortunato behind a wall of bricks, and kill him. When he pulled it out of his coat, the reader could predict what he might use it for.