Reserve irony for situations where there's a gap between reality and expectations, especially when such a gap is created for dramatic or humorous effect.
In Greek, eiron meant "a dissembler" — someone who hides their true intentions. Today, we do something very similar when we employ irony, often by saying the opposite of what we really mean. The word can also refer to a situation that turns out to be amusingly different from what we expected: "I thought he had stolen the Fig Newtons, but the irony was that he thought the same thing of me." Note that this is more than just an improbable coincidence!