If you say something irrelevantly, you say it in a way that does not relate to what is going on, so it does not help and is not important for that moment. It often describes a remark that comes in from the side: she added, irrelevantly, that the cat was hungry. The describing word irrelevant is much more common; this -ly form is a bit bookish and shows up in stories and reports. Though actually, people sometimes call a comment irrelevantly when they only mean it was badly timed, not that it had nothing to do with the topic.