Most people confuse coincidence and irony.
In Broad Strokes
Coincidence occurs when seemingly unrelated events happen by chance, simultaneously. As improbable as it might be, it is simply an alignment of circumstances. Imagine it raining on your wedding day. Though you may not like it, this random event is a coincidence.
Irony, on the other hand, is more nuanced. It may contain elements of coincidences; however, it is rooted in opposites. For example, imagine that you look at the 5-day forecast, and it shows that it is supposed to rain on your wedding day. So, you move your wedding to the day before, when it is supposed to be clear and sunny, only for it to rain super hard on your new wedding day and then your original wedding day turns out to clear and sunny. That is irony, and you are probably going to get a divorce.
Determining Irony versus Coincidence
Intent
Coincidences have no intent behind it; it’s the result of random chance. Irony often involves intention; a deliberate use of language or action to convey an opposite meaning.
Expectation
Coincidences defy our expectations because they’re unexpected. Irony, on the other hand, relies on expectations; it plays with what we anticipate versus what actually occurs.
Emotion
Coincidences can evoke wonder or surprise and can carry a lighter weight than irony (e.g., serendipity, fortuity, and luck). Irony often carries a tone of a deeper meaning or a darker humor (e.g. satire, schadenfreude, mockery).
Frequency
Coincidences are relatively common; they happen to everyone, sometimes daily. Irony, especially well-crafted verbal irony or situational irony, is less common and often requires a certain level of thought, wit, and even creativity.
Want More?
Dive deeper into irony and its many mistaken similar concepts from coincidence to paradox in this MIT Press article by Roger Kreuz, “What Irony Is Not.”