the usual comment re Druids is that what is known can be written on the back of a postage stamp - much of this is due to Caesar (or those who supplied him with info) and is hostile - it is not certain that the same practices whatever they were , were uniform across the Celtic world - eg in parts of Europe there would appear to be temples with niches for heads to be displayed - the skulls found appear to be of young males. What most think of as Druids, including the garb etc and bardic chairs , is a complete fabrication of the late 18th C.