In "The Cask of Amontillado" what was ironic about the setting of the carnival?

The carnival was a time of great merry-making, partying, costumes, revelry, drunkenness, and mirth.  To be plotting someone's demise amidst all of that is most unexpected.  You would expect such a creepy tale of revenge to immediately start off with the more appropriate setting of the cliche "dark and stormy night", not at a happy festival of people.  So, the irony exists in the fact that that setting is almost the exact opposite of where they end up, and of what Montresor's dark mind and plans are.

It works well in the fact that first of all, Fortunado is dressed up in a ridiculous costume, with a jingle-bell hat on.  It makes his end a bit more pitiful; Montresor pokes and prods in the hole only to hear a few jingles of bells at the end.  It is a comical tragedy, almost, to picture Fortunado in that completely unsuited outfit at his death.  Also, because of the carnival, all of Montresor's servants "had absconded to make merry in honour of the time", so there were no witnesses to him bringing Fortunado into the catacombs.  Very lucky indeed.  Then, because of the carnival, Fortunado is drunk, which makes Montresor's plan that much easier; he is more willing to follow, less alarmed, a bit hazy, and doesn't quite get what is going on fast enough.  So although the carnival is quite ironic for the morbid tale about to unfold, it works very much in Montresor's favor.