Cultural groups reappear in so many countries that it is helpful to isolate and follow their migrations.
The expanse of Celtic culture is known, but its origins are more obscure. It appears, however, that the Romanized Celtic era of some 500-400 BC is late. Earlier is the Celtic culture of 1500-1000 BC, in eastern France, to somewhere south of Geneva, and parts of Western Germany, and what is now Luxembourg. See Celtic Guide, Historical Background, at http://www.siliconglen.com/celtfaq/1_1.html. It was from that group, that the Celts so familiar in the British Isles and other parts of Europe evolved.
Germanic and Roman cultural groups pushed them -- also known as Gauls in France by the time of the Roman legions -- off the continent into refuge areas in the British Isles.
With the mountains there, Luxembourgers -- how much Celtic is left? -- survived, see Luxembourgers, Development of a Nation, at http://www.worldology.com/Europe/Europe_Nations/Luxembourg/index.htm