The history of Druidism is odd. For a tradition which boasts of its antiquity with several ancient orders of the druids around, the history of Druidic practices does not go that far back. Though Caesar and Tacitus mention them, as Ronald Hutton explains in his latest book (and Tom Shippey in a review of it in the London Review of Books concurs), the ancient evidence for them as sparce. It could be put together in about a dozen pages. There is no archaeological evidence to speak for. There are almost no monuments that can be associated with druids (Stonehenge is much older than the first mentions of druids that we have). Furthermore there are almost no Celtic traditions about the druids- these are all medieval and there are no medieval references to medieval druids, if there was a druidic tradition it managed to avoid Malory, Geoffrey of Monmouth and all the chroniclers of medieval Britain. Even the ancient evidence that we do have is unreliable- Pliny's writings about them are credulous, Caesar's justify Rome's imperial mission- a couple of pages here and there, introduced for local colour, by authors whose true focus is not Britain but Rome does not give us much to base a history of druidry upon.
What Hutton and Shippey argue therefore is that it is much more interesting to look at modern druids. Think of the analogy to King Arthur- there is almost nothing that we can know about the historical King Arthur but the modern King Arthur is an eco campaigner who has done interviews. Ultimately the two historians here are more concerned about the reflection of whatever ancient druidry was upon the modern and early modern world. What they suggest is interesting- Hutton's book, and Shippey accepts his conclusions, suggests that the druids were part of an invented tradition of welshness, created by the talented Welsh immigrants who streamed into London in the early modern period in the eighteenth century. This invented tradition was buttressed by the insane, the fraudulent and the fantastical until it became what we know today as druidism. Druidism has nothing to do with ancient druids- but it is interesting because enough people beleive that it does have something to do with them and that this is important to make it important in the modern world.