"... some similarity between the Ainu and the Celtic language in respect of places names: 'petsu' in the Ainu language and 'perth' or 'firth' in the Gaelic language are phonetically similar."Though it has been speculated, it is highly unlikely that the Celtic culture is related to the Ainu culture (Fukasawa, 1998).
Fukasawa, Yuriko
1998 Ainu Archaeology as Ethnohistory: Iron technology among the Saru Ainu of Hokkaido, Japan, in the 17th Century. Oxford, England: British Archaeology Reports.