The Druids were once an elite, spread across the British Isles and Gaul. They did not directly rule. However, they were responsible for laws, culture, medicine, and interactions with the Gods; they could and did exclude transgressors from these areas, and so they were listened to and respected by all levels of society, exempt from taxes and military service, and able to intervene to stop all conflicts. Their greatest centre of education and learning was on Anglesey, but there must have been other schools in other places. We don’t know where their revenues came from but they must have been substantial to support this kind of system.
After the Roman conquests of Gaul and Britain, druids entered a new phase. The Romans had destroyed their power structure, as well as their ‘university’ on Anglesey (though probably incompletely). Many of the druids were killed (though many must have fled to northern Britain, and to Ireland, or gone underground). Rome commanded the areas of law, and commerce, and its military might compelled the loyalty of all levels of society in the area under their rule. Druids were proscribed throughout the empire.
During this period, and during the post-Roman period in Britain, druids continued to exist, and teach. However, instead of being a separate, independent, super-tribal elite, they were now dependent on individual tribes and kings for their support, and so became counsellors to their patrons – who existed in the Old North, the Welsh-speaking kingdoms north of Hadrian’s Wall, where tribes were Romanised, but still politically and militarily independent. Bards were probably still free to wander where they liked, being less committed to one king or the other. Also, it seems likely that there would have been less opportunity to pursue astronomical observations, discuss philosophy, etc in this context, so it may be that the “druids” of this period were more like the “ovates” of the pre-Roman golden age.
In Gaul during this period, the proscription of Druids seems to have fallen into disuse; it seems that Druids also continued to exist on the continent. However, in this very different socio-economic environment, there were no independent kings to support them, so they must have had to support themselves – hence the record of a future Roman emperor taking advice from a druidess who ran a tavern.