What is ironic about the appearance of the Red Death to the abbey?
The appearance of the Red Death in Prince Prospero's abbey becomes the ultimate irony in Edgar Allan Poe's "Masque of the Red Death."
Prospero and his favorite courtiers "retired to the deep seclusion of one of his castellated abbeys." Prospero saw his escape as a grand adventure with "all the appliances of pleasure."
The irony of Red Death's appearance, "his vesture [...] dabbled in blood -- and his broad brow, [...] besprinkled with the scarlet horror," is two-fold. He arrives in the middle of Prospero's elaborate ball, and Red Death's costume is actually a representation of the grisly symptoms of the dreaded plague. His appearance also results in the subsequent deaths of all the inhabitants of the Abbey; the irony of this development is that the sanctified walls of the Abbey that were supposed to protect actually become that of a trap, sealing the desperate courtiers inside like a crypt.