What was the irony in the story the cask of amontillado?

Irony (from Ancient Greek εἰρωνεία (eirōneía), meaning "dissimulation, feigned ignorance), in its broadest sense, is a rhetorical device, literary technique, or event in which what appears, on the surface, to be the case, differs radically from what is actually the case. Irony may be divided into categories such as verbal, dramatic, and situational.

The meaning of the word in this sentence is unneeded