When did the Celts dominate Europe?

The Celts never dominated Europe as they were not one people in the accepted sense, rather were they loosely formed tribes.They are variously described as Iberian or Berber Celts or Baltic Celts, either very short and swarthy or abnormally tall and ginger haired which tends to suggest that the early writers about them -the Romans were mistaking them for another group of people altogether, possibly a vanished Germanic tribe.The Iberian Celts similarly have vanished as a tribe or tribes, but some say that the dark, swarthy inhabitants of the Scottish highlands and small pockets on the Irish West coast islands resemble their desert forebears who came from North Africa.

Answer

Many people get confused over this. Calling the gauls Celtic etc. But they were a tall Germanic people not Celtic. The Berber Amazigh Celts from north Africa never got across Europe. They made it to Iberia / Basque / Southern Ireland.