From the Celtic Christianity List FAQ:
2.1 Who were the Celts?
The Celts are a cultural and linguistic group that currently are
centered in the British Isles, and France. There are also many Celts
around the world today in what is called the Celtic Diaspora. If you
have Breton, Cornish, Irish, Scots, or Welsh ancestry, you are a Celt.
Our ancestors can be traced back to about 800 bc. in Eastern Europe,
from there they spread throughout Europe, Asia Minor, and the British
Isles. They reached the height of their power around 400 bc. when they sacked Rome.