Well, first let's talk about what situational irony is and then figure out how it's present (in many ways) in The Visit. Situational irony is present when an expected result is different from the actual result in a story. Using this definition, there are many instances of situational irony in The Visit.
Much of the situational irony revolves around Claire's visit to Gullen, her poor native town decades after she had to leave in disgrace. The first bout of situational irony is that the townspeople expect the financial gift from the (now rich) Claire to be given without strings. The actual result is that Claire comes with riches for the town with a caveat: kill Alfred III for the dishonor that he has inflicted upon her.
Another tiny bit of situational irony is that while Claire is waiting for the townspeople to take action against a former lover she marries numerous more times. This helps to prove the true insidiousness of her actions in the first place.
Another large incidence of situational irony is that Alfred III ends up owning up to his actions on his own: Alfred admits he has had Claire as a former lover which produced the child who lived only one year (the reason for Claire's former disgrace). This is unexpected, as Claire has simply desired for Alfred's murder as "justice."
Finally, because Alfred admits his own disgrace, we expect that he might commit suicide instead of being killed. In fact, the townspeople hope for this because then they won't have any guilt associated with his murder. What ends up happening is that the townspeople are forced to murder him anyway in order to get Claire's promise of money.
In conclusion, it's important to note that the last incidence of situational irony is also an incidence of dramatic irony (when the reader knows something a character does not). By the end, the reader knows Alfred is planning to refuse suicide. The townspeople have to murder Alfred as a group in order not to incur solitary guilt.