"taking over most of Europe as Celts used to live all over France and beyond." If historically it had belonged to the Celts, and all the land had been called I

"taking over most of Europe as Celts used to live all over France and beyond." If historically it had belonged to the Celts, and all the land had been called Ireland since antiquity, and was currently controlled by Ireland and all the citizens considered themselves Irish then yes, Ireland should control most of Europe.