What is the irony in the name Fortunato?
Giving this character the name Fortunato was intended to indicate that he was both fortunate and unfortunate. He was fortunate because he was born into a wealthy family and had an easy life with plenty of money to spend and lots of friends. Montresor explains this in the story.
“Come,” I said, with decision, “we will go back; your health is precious.You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—”
The irony in the author's choice of Fortunato's name is that this unsuspecting man is about to be very unfortunate indeed. In fact, it is hard to imagine how a man could have anything worse happen to him than being chained in a dark niche underground, walled up, and left to die of starvation, terror and madness. The fact that he had been so fortunate before that awful event only serves to make his fate that much more gruesome by contrast. He dies wearing the colorful jester's costume and the hat with little ringing bells. He was having a great time at the carnival until Montresor enticed him into his trap.
Life had been easy for Fortunato up to that point. The main difference between the two men is that one inherited a lot of money while the other is poor and has to struggle for to stay alive. Throughout the story Fortunato and Montresor sound like rich and poor men, respectively. Montresor is consistently polite, obliging, humble and obsequious. Even when Montresor is leaving Fortunato to die, he talks to him with pretended courtesy and consideration.
“Pass your hand,” I said, “over the wall; you cannot help feeling the nitre. Indeed, it is very damp. Once more let me implore you to return. No? Then I must positively leave you. But I must first render you all the little attentions in my power.”