Trying to put national borders around "Celts" or "Jews" or any other group is perilous, and ultimately not very important, IMHO. My interest is in trying to identify the major streams of history insofar as they apply to the ancient Israelites.
It is not difficult to identify the Celts as the rootstock and main contributors to the genetic pool of most of the nations of NW Europe. And it is becoming increasingly easy to identify the Kingdom of Israel, known as the Lost Tribes of Israel taken captive to Assyria as the source of the Celts. Every year brings more archeological finds which reinforce what were hundreds of years ago just "hunches" or theories. A major NYT article last December reported the Celts in eastern Turkey about the same time the Lost Tribes went missing in the same area. When such coincidences become routine, maybe it isn't just coincidence any more.
Much of European history is about these Celtic groups, The Lost Tribes, fighting among themselves, just at they did in Egypt and ~Palestine long before.