What is ironic about the toast of Montresor in "The Cask of Amontillado"?

The toast that Fortunato and Montresor share is as follows:

“I drink,” [Fortunato] said, “to the buried that repose around us.”

“And I to your long life.”

Fortunato's toast is ironic because he will soon be among the dead buried in the catacombs himself. Montresor seems to be wishing his companion a long life as a typical toast suggests, full of heath and happiness, but Montresor hopes that Fortunato will live a long time chained to wall and suffer before he dies in payment for the "insult" he dealt to Montresor. His response, therefore, is ironic as well.