If you're talking about hockey and someone keeps trying to mention football, you might describe their comments as irrelevant. Irrelevant means not related to the subject at hand.
If a rock star becomes irrelevant, it means people are not relating––or even listening––to his music anymore. It isn't part of what people are thinking or talking about. The opposite is relevant, meaning related. An analysis of brush strokes in Manet's painting would be relevant to a discussion of texture in painting in general. Relevant was borrowed from Medieval Latin relevare "to bear upon," from Latin, "to lift up." The prefix ir-, meaning "not," is a variant of in- before words that begin with the letter r.
