Irony of Cunning
The evil Sicilian mastermind Vizzini, having successfully convinced himself that he knew which glass of tea was poisoned, fails to consider that perhaps he doesn't understand the basic assumptions of the game he is playing. He believes that if he correctly guesses which cup contains poison, he will survive the encounter. In reality, the Man in Black, who the reader understands to be Westley, has no interest in matching wits with him and is simply out to kill him. He has poisoned both cups of tea, having spent the past few years building up an immunity to the deadly iocane powder.
Irony of Revenge
Iñigo Montoya spends his entire life training to kill Count Rugen, the swordsman who murdered his father. He succeeds in doing so despite being severely wounded; however, after he realizes that killing Rugen won't bring his father back he does not know how to live a life that does not revolve around revenge. "Revenge", incidentally, is the name of the Dread Pirate Roberts's ship. Westley indicates that he may retire after reuniting with Buttercup.
Irony of Marriage
Buttercup believes she is being forced into a loveless marriage with Prince Humperdinck, however in reality he is doing his best to sabotage the marriage. The Prince arranges Buttercup's kidnapping and plans to murder her so as to provoke a war with a neighboring country.
Irony of Subversion
After spending an entire novel showing the myriad ways in which life is not like a storybook, subverting and poking fun at the reader's expectations for an adventure story, the author allows the two star-crossed lovers to get back together. He allows a dead man (Westley) to be reanimated, and everybody escapes.
Irony of Fairness
Every time a character tries to behave in a fair manner, he ends up regretting it. Iñigo, for example, allows the Man in Black to rest after climbing up the cliff so that the fight can be "fair", and is defeated for it.
