The word "Celt" concerns only a linguistic notion, and refers to a linguistic group, as the Germanics or the Slavics. Every other consideration is vague and non-scientifical.
If we consider that indo-european peoples have come from the East by one or several waves, we have to admit that the non-indo-european groups as the Ligurians, the Aquitanians, the Etruscans...have necessarily preceded these waves. And the fact that some Celtic superficial features can be found in those cultures does not change anything.
It's a stock of knowledge....terrific dude.