How We Know What We Know About Druids:

Another micro-essay, stored from a FB post so I don't ever have to type it again ; )

1: Classical Commentators. Greeks traded wine for salt with central-European Celtic-language peoples (Gauls). Romans encountered Gauls as early as 400 bce. Several key accounts by early Greeks (i.e. 

Julius Caesar writes about the Gauls and Druids. Current scholarship considers him a reliable source on the topic, though he often merely quotes older sources.,(Poseidonius) become references for many later Greek and Roman writers.

Typical Victorian Druid-depiction.

2: Insular Literature (insular means 'from the islands'). The arrival of literacy in Pagan Ireland led to the writing-down of various bits of lore. Famous works include the Book of Invasions (story of the coming of the gods and men to Ireland); The Tain Bo Cuailgne (Cattle Raid of Cooley - the story of the hero Cuchulain); The Dinnsenchas ('place-stories' - the stories of how places in Ireland got their mythic names; and the tales of the hero Fionn MacCumhal (Finn MacCool). Later, medieval authors wrote literary versions of British ("Welsh") stories. These are contained in the "Mabinogion" (Childrens' stories) and include the earliest tales of King Arthur.

These sources preserve a wealth of lore, but should all be examined for Christian, Biblical and Classical influences. While they contain bits of "Celtic mythology", the stories in them are certainly edited by monks.
Note that in this I am, as always, making the word Druid mean primarily "Celtic Pagan priest". Mythology is 'about' Druidry in that it would have informed the concepts of the divine.

3: Archaeology. The effort to establish the facts of material life in the Celtic Iron Age has provided confirmation and support of a great deal of the first two categories, while casting doubt on other portions (there never seems to have been chariot warfare in ireland, for instance...)

Corn-dollies. Like Shinto paper-folds, we
haven't the slightest idea what these mean.
Sure feels Pagany, though.

4: Early-modern folklore. Gaelic, British and even Franco-German countryside custom in the 18th and 19th centuries contained ritual, song and story that preserved ancient elements. Versions of the Cuchulain and Fionn stories remained in oral tradition into this period. 

At the end of the 19th and beginning of the 20th centuries it was fashionable to believe that folk customs preserved truly archaic elements down through many centuries, so these customs were looked to as sources for old ways. Current scholastic fashion is less confident of the memory of folk tradition, noting frequent innovation and re-strats. Still, when folk custom matches the previous two sources, one is on to something.

5: Comparative cultural parallels. Celtic peoples were part of the spectrum of languages, cultures and religions that are called 'Indo-European'. They shared cultural structures, vocabulary ('bo' is 'cow' in irish or latin...), artistic forms and, almost certainly, religious and ritual forms as well. Again, when something from the first three sources echoes a motif present in neighboring cultures one can be confident of a 'hit'.

Ritual in the Baltic cultural style. If these folks don't look
like Druids, I dunno what does.

That's the deal. Everyone from Elias Ashmole onward has been drawing on these same sources mixed with whatever bit of esoteric or Masonic fashion influenced them. Academic scholarship on all four topics has improved dramatically since the 18th century - modern readers can know so much more than those of former centuries. Likewise we have been freed from established religion - we are free to worship as we will, without hiding under the cloak of 'fraternalism' or 'philosophy'. Unless we like those, of course...