This can have both literal and ironic meanings: Fortunato was of the false aristocracy, which implied that, while not born into wealth, was able to somehow acquire such social standing, and was, therefore, of good fortune. Ironically, his fortune had run out that fateful carnival night when he ran into the narrator. Montresor, or "my treasure", has only ironic meanings to his name. While he apparently still maintains the true wealth of his aristocratic family, he has lost the social rank that was once part of his family line. While this treasure is lost, he regains another treasure: a dead rival, whom he blames for his family's fall, sealed in the family vaults.
There are many other instances of irony in "The Cask of Amontillado.".
Fortunato, a wine connoisseur, is buried in a wine vault. More ironic, this vault is the burial catacombs of the family that he has insulted. When given a glass of wine by Montresor, Fortunato drinks to "the buried that repose around us", shortly before joining those dead souls (Poe 343). Montresor also makes a toast, one to the long life ahead of Fortunato. In committing this perfect crime of revenge, and robbing Fortunato of his life, Montresor also robbed himself of his own humanity, feeling no remorse for his crime. Also there is the simple irony that while Montresor greets Fortunato as a friend, he had every intent from his first word to his victim that night of burying Fortunato alive in his family catacombs. As stated by John Freehafer, early on, Montresor refers to his purchase as a "pipe", which is able to contain four barrels of wine, which is large enough to be a coffin for poor Fortunato (316). Despite Montresor's strong determination to uphold his family motto, "Nemo me impune lacessit" (no one insults me with impunity), his heart still "grew sick" with grief, though he denies it, attributing the feeling to the catacomb's dampness (Poe 343).
Symbolism is another major element Poe employed in this masterful story.