I consider the druids to be figures of mythology. I don't know for a fact that they didn't exist in reality, but I also don't see much evidence for their existence as actual historical characters as they have been described in the histories written by their purported opponents. If the druids really did exist, then it seems that they could have been dramatically different from what hearsay has made them out to be.
As it is, there is no historical evidence to give us concrete clues about the life of the druids.
That's the quandry explored by the writer at Irregular Books, who writes: "What do you do when you want to write about a group of people whose history has intentionally been left in mystery?"
The answer offered is an uncertain choice: Write about the druids as fiction or don't write about them at all.
Are there other alternatives available for writing about legendary figures for which there is no concrete evidence?
An answer that may not satisfy the modern obsession with fact is that the concept of druids satisfies the human need for mystery. By being shrouded in the fog of lost preliterate culture, the druids have taken on a significant that is able to shift with each passing generation. Precisely because the druids are so little known, they are deeply fascinating.
Consider how you might feel upon learning about precisely what the druids were up to in their meetings in the groves. Were they passing bylaws? Crafting mission statements? Accounting for the harvest of mistletoe? Certainly, reality could not be as interesting as the shadow under the druid's hood.