They'd earlier come from the Black Sea area, on the Volga, upstream of what is now Bulgaria. Everything along that river has a Celtic name you know, and almost everthing on the Mediterranean shore West of Siciliy also has a Celtic name.
The ancient stories (written in Greek) suggest that the Celtic conquerors of Ireland took neighboring people with them as slaves. It's for that reason that the Irish and the Basques are now considered the exact same ethnogype. However, the Celts were the leadership elite and they moved on to Britannia.