Irony is one of Poe's many stylistic triumphs in his body of works. This story is no exception.
Your first example of Fortunato's toast to the dead is ironic because of the nature and tradition of toasts. People make toasts to celebrate happy occassions such as marriages, birthdays, graduations and promotions. They are made to give encouragement and to wish luck upon very living recipients. The dead do not have any use for luck or for encouragement. The only two living people there are Fortunato, who will soon be joining the dead, and Montressor, who will be killing him.
The second example is the pride of the knowledge Fortunato has of good wine. The rare cask draws him toward his death. It is his own pride in wanting to see this rare gem, or perhaps to prove Montressor wrong, that lures him to his doom. Pride is a classic tragic flaw which has caused the downfall of great men in literature and in life.
"May he rest in peace" are the words of Montressor as he is concluding his tale of revenge to an unknown audience. He shows no remorse at the cruel death he has inflicted upon Fortunato and utters these words knowing that Fortunato will NOT be resting in peace. In fact, he will be dying a very slow and frightening death cemented alive in the walls.
Some examples of irony in the story are Fortunato and Montressor's name, Montressor's coat of arms, and "I shall not die of a cough".