> The Celts did in fact inhabit most of Europe in pre-Roman times, or at least very big swathes of it. For example there are some stone-made Celtic-crosses that are popular in what is today North-Western Romania
Celtic crosses are a Christian thing and Christianity only got started after the Romans had subjugated large parts of Europe, so any "Celtic" crosses in Romania are guaranteed to be unrelated to a possible pre-Roman presence of Celtic tribes. (And it's not like putting a cross in a circle is a terribly original idea, so they probably came up with it independently from the Celtic Christians in Ireland.)