Something is irrelevant if it's not connected or useful for what we're talking about. It might sound obvious, but that's because it often feels true.
Imagine someone trying to convince you to buy a new car with all sorts of features, but then they tell you that the car can hold 20 people inside. If you don't need a vehicle for 20 people, that detail is irrelevant - it doesn't matter for your decision.
Not related, not applicable, unimportant, not connected.
