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.
As Fortunato and Montresor are descending into Montresor's vaults, they enjoy a draught of Medoc to ward off the cold. Fortunato says, "I drink..to the buried that repose around us." Montresor responds with his toast: "And I to your long life."
Montresor's toast is ironic because it is his intent to bury Fortunato in his vaults. They continue to descend lower and lower into the Montresor family catacombs until they reach a crypt piled on three sides with human remains. In the fourth wall, there is another internal crypt, and Montresor lures Fortunato into this crypt with the promise of showing him the cask of Amontillado. Then, he fetters Fortunato to the granite and builds a wall to enclose the crypt. There, Fortunato will be buried. Therefore, while Montresor is toasting his friend's long life, he plans to soon have his friend meet an untimely demise in his crypt.