Because the story is told from a third person point of view and because it could be interpreted that the narrator could be the "Red Death" itself OR the Prince himself (meaning the prince has dreamed ALL of this tale and the death is his own psychological death), the entire story could be seen as an example of dramatic irony.  Dramatic irony occurs when the reader knows more than the character knows, so if the narrator is either the "Red Death" or Prince Prospero, it is told AFTER the fact, which would make the entire story an example of dramatic irony.