Druids have become, in the modern lexicon, a symbol of Celtic magic or mysticism. Part of that is the fault of the early Christians in Britain and Ireland; they portrayed druids as the keepers of the old religion who had to be outshone in order to usher in the new; Columba defeats a Druid several times in bringing Christianity to Brude, king of the Picts. No doubt stories abounded about Patrick accomplishing the same feats in Ireland.
Part of the problem also has to do with our perceptions of magic. Today we see the craft as the province of people who study for years, people who go by the names of witches and wizards. But that was not the case in the ancient and medieval worlds. As has been seen, bards were wordsmiths, and for that reason their creations had a supernatural quality. The king’s power over his people rested in the divine provenance of a deity or the land itself. A talented smith could put magic into his weapons and armor. Magic was not limited to those who studied the supernatural, but to those who studied their craft to a high degree of precision.
There is a limited amount of information about druids, and much of that is either second-hand or contradictory. They liked oaks and mistletoe, probably because of the male symbolism of both plants. They were respected among all the Celtic tribes regardless of where or to whom they were born. They often acted as diplomats. The pre-Christian Romans feared them. That’s about all we know.
But that last bit of information is perhaps the most interesting. The Romans accepted all religions up until the deification of their emperors, and even then they only insisted that all peoples in the empire worshipped the dead emperors as a part of their native pantheon. Only three groups had issue with that law. The Jewish people would be forced to scatter as a result into their diaspora, the Christians would be persecuted within the empire until Christianity became the state religion, and the druids who were wiped out.
Notice that the Celtic people were not persecuted. In fact there is no record of any Celtic people suffering under Roman rule, apart for the druids themselves. Does that mean druids had their own separate religion, perhaps monotheistic? Could they have been more politically influential than our limited records suggest and have been fomenting some massive rebellion? No person really has any idea. All we can be certain of is that in the first century, the Roman army went to the island of Anglesey in Britain and there slaughtered the druids. That is the last time any druids living south of Antonine’s Wall were ever mentioned in any source.