Leather hulled wood boats after 4,500 years? If one got buried in a peat bog it might be found intact, but otherwise little or no chance. There’s ample evidence of migration and trade from Europe into Ireland and Britain even earlier than this, but it consists of the usual kinds of durable artifacts like pottery archaeologists customarily deal in.
I dunno about replacement theory. That requires genocide. They have no evidence of that. Otherwise, despite invasion, the invaded still remain. That’s been my experience.