The term has no meaning to me at all. The people of Ireland are Irish and the people of the central European lands formally occupied by the Celts now have a variety of national names - Austrians, Swiss, French etc.

If you want to believe that the Iron Age people of Ireland were Celtic then that's up to you. You'll not be alone in that fantasy.

Read around the subject and make your own mind up - I'll not try to convince you because I'll only be repeating what I've written already.
Good luck with your studies.
Peter