https://en.wikipedia.org/wiki/D%C3%A1l_Riata
NB I have stood on Dunadd with my foot in the Footprint of Fealty where kings were crowned:
I've found another blog-post with more photos about the Celtic crosses from Maramures [3], and in the comments in there someone also linked to similar crosses being found in Northern Greece/Macedonia, close to Thessaloniki [4]. Very interesting stuff.
[1] https://www.bzi.ro/morosenii-au-origini-celtice-artefactul-c...
[2] https://www.churchofscotland.org.uk/news-and-events/news/201...
[3] https://www.atlasobscura.com/places/abandoned-bogomils-grave...
[4] https://www.atlasobscura.com/places/abandoned-bogomils-grave...
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.)