I'm not going to do an entire report 2000 years of Gaelic history. "Gaelic" "Irish" and "Scottish" were interchangeable until about 1500. They were ethnonyms not nationalites (there was no united "Irish" nation to begin with). The Pope declared "Scotia" should refer to only the Gaelic speaking people in what is now Scotland (who originally came from Ireland, which was called Scotia during Roman times) and that stuck. Anglicized Scots appropriated "Scots" as an ethnonym from Gaelic speakers and started referring to them as "Erse" to try and portray them as non-native. If being "Irish" stopped soon as you got on a boat Scotland would be Picitish or Welsh, as would England (since Angles magically transmuted into Brythonic proto-Welsh speakers when they invaded). https://en.wikipedia.org/wiki/Gaels