The irony is that Fortunato is completely unaware of Montresor's hatred of him. Though it is never explained what he did to anger the man, Montresor is bent on revenge.
Fortunato is not exactly the sharpest knife in the drawer, if you know what I mean. His naivety is easily exploited by Montresor. He drinks too much wine, plied liberally on him by his enemy and this doesn't help sharpen his senses.
Montresor lures his unsuspecting prey into his catacombs where he stores his wine. He tells Fortunato that a rare wine can be sampled there. Too excited, simple-minded and intoxicated, Fortunato is led through the catacombs and to his grisly death.