Final answer:
The narrator's toast to Fortunato's long life is ironic because it contradicts the narrator's true intentions of leading him to his death.
Step-by-step explanation:
The irony of the narrator drinking to Fortunato's long life lies in the narrator's true intentions. This irony highlights the juxtaposition between what the narrator says and what he is actually planning to do — which is to immolate Fortunato. The toast to his long life is bitterly sarcastic because the narrator leads Fortunato to his death through his eagerness for fine wine. This act of treachery is veiled by a facade of goodwill and the lure of the Amontillado, which ultimately leads to the dark culmination of Fortunato's fate in the catacombs.