I am familiar with the Norse expansion into the Atlantic, and the notion that a 'Celtic' people travelled to the Americas before the modern era is a pretty old myth, occasionally refuelled by people placing hoaxes in the Americas themselves. It is a consequence of Welsh, Irish and Scottish immigrants to the Americas, and is part of the formation of their national identities during the 19th century. There is no archaeological evidence for such a thing, and in its earlier incarnations the idea was heavily racist, coupled with the idea that all 'hallmarks of civilisation' (mostly durable stone architecture) could only have been brought to the Americas by white Europeans.