@JohnBrady @chrisaldrich You are right, the Celts seemed to be everywhere for a very long time: they sacked ancient Athens (if I remember right), they sacked Rome long before the Germanic tribes were even around. And of course, the Romans never forgave them for that.
What always struck me about the Celts is that they could never truly unify for very long even against an overwhelming threat like the Romans. Once in awhile they could come together under a warlord like Vercingetorix but it never lasted long. And this pattern repeats itself from Caesar's campaigns in Gaul to Medieval times of the Welsh vs. the Normans and later the English vs the Irish. Wales had many small kingdoms and principalities and Ireland had many kingdoms (with a temporary High King for emergencies) but neither could really unify.
So the Celts remained tribal throughout their long history and seemingly just as happy to war against each other as they were to fight non-Celtic neighbors. Anyway that's my sloppy, broad brush take on it.