This is an example of irony because Montresor is toasting to Fortunato's long life, but Montressor later kills Fortunato, so he doesn't live a long life.
No one attacks me with impunity
"The drops of moisture trickle through the bones." This sets an eerie and dark mood. The mood is also set by where it takes place-the catacombs.
Fortunato is dressed as a clown. This represents his personality:silly, a fool, and a joke.
Fortuanto previously insults, (attacks) Montressor. This is irony because Montressor tells Fortunato what is basically going to happen to him (murder or revenge) and Fortunato responds with "Good!"
When Montressor pulls out the trowel, it foreshadows what he might do to Fortunato later, (chain Fortunato to a wall and use a trowel to build up a wall).
The catacombs symbolize Montressor and his personality. They are dark, twisted, and eerie. The final action against Fortunato by Montressor represents just how dark and twisted Montressor is, just like the catacombs.