The first is when Mrs. Peters, who is clearly in a state of distress, asks Officer Warren if he can find her some whiskey to calm her nerves. He responds that he has no whiskey and has never heard of anyone needing whiskey to calm their nerves. This irony is compounded by the fact that the audience knows that Mrs. Peters has just learned that her husband has been murdered—and yet she still needs a drink!
Another example is when Mrs. Peters asks Officer Warren what she should do with the broken pitcher and glass. He responds that he doesn't know anything about them or how they got broken—but then goes on to say that, if he had to guess, he'd say they were broken by some man who was trying to defend himself from an angry woman armed with a hammer! This is ironic because it's clear from the way he's talking that Officer Warren doesn't believe this theory at all—he just wants to make sure Mrs. Peters doesn't get any ideas about taking revenge on whoever did this terrible thing!
Situational irony occurs when a character's actions are incongruous with what he or she intends to do, or when a character's intentions lead to a result that differs from what he or she intended. In this case, Mrs. Peters and Mrs. Hale try to hide the evidence of the crime they believe has been committed against Flora, but their actions actually reveal it.