From about 500 - 100 BC the Celts dominated most of Europe. (around 400 BC was when they defeated Rome and besieged the city until the Romans paid them to leave).
Around 200 BC their domain reached from Ireland to Turkey and from Spain to Holland.
Because of the growing strength of the Greeks, Romans and Germans, together with the fragmented nature of the Celtic empire it shrank and was eventually obliterated (yes, obliterated) under Julius Ceasar's command circa 60 BC.

I'm surprised such obviously knowledgeable people as yourselves don't know all this already.

John