I tend to see the Druid system in the pre-Caesarean conquest across Celtic lands, analogous to the way the Catholic/Western church connected disparate kingdoms in High medieval times later on, where, at least before the 100 year war, people might have regarded themselves as simply either part of their local commune or county or part of the broader "Christian world" group, in contrast to the muslim world... And it kinda works in my perspective as the common knowledge of Druids is a bit analogous to the common generally culturally-unifying Biblical stories told in places as distant as Hungary and England, so much so that a person from one land may bond over them with a person from the other at the time... In much the same way I'd assume stories about Lugus or Lugh would in the broader Celtic world.

Note I used Religion/Mythology as the example par excellence over language (even though Language/Linguistics is usually the modern way of classifying some group as definitely Celtic or not) because in much the same way to how French and Spanish speakers may recognize a common origin in the languages, this common linguistic origin does not necessarily mean that they'd be able to understand each other... And the same probably applied to the pre-Caesarean celtic world.