If Fortunato's name is anything to go by, we might expect him to encounter nothing but good fortune. “Fortunato” is the Italian word for fortunate, and so we might think that Poe chose to give him this name to give us an insight into his character.
As it turns out, however, our expectations are thwarted, and herein lies the situational irony. The exact opposite of what we'd expect to happen—Fortunato living up to his name and being lucky—happens, and poor old Fortunato is most unfortunate in being walled up alive in the catacombs by the wicked Montresor.
Further situational irony comes from the jester costume that Fortunato wears for the carnival. Once again, our expectations are confounded. We might expect—as indeed might Fortunato himself—that the carnival would be a fun occasion full of revelry, enjoyment, and laughter. All of this wonderful jollity is symbolized by the jester costume that Fortunato wears.
And yet, once again, situational irony enters into the breach as the fun that Fortunato expected to have turns out to be the exact opposite. As Fortunato isn't some kind of masochist with a death wish, it's safe to conclude that there's nothing remotely fun about his being walled up alive inside the catacombs.