The word 'Druid' was one given to experts in magical and religious practice by the peoples speaking Celtic languages who inhabited northwestern Europe around 2,000 years ago. That is all that can definitely be said about it. Those who have tried to say more have relied on two different groups of sources. The smaller, but more famous of those groups consists of the writings of ancient Greeks and Romans. These have the virtue of being the work of people who lived when Druids still existed. Their problem is that almost all relied on secondhand information of unknown quality, much of it very old even by their time. Moreover, none wrote more than a few sentences about Druids
The only one of these writers who could have encountered them himself was Julius Caesar, who conquered Gaul--present-day France, Belgium and the Rhineland--for the Roman Empire. In a famous passage he describes the Druids of Gaul as having great power and learning and being united in a national organisation under a single leader. No other ancient author credits Druids with this degree of sophistication. Furthermore, his famous description of them is isolated amid detailed accounts of the wars in which he conquered Gaul. If the Druids had been anything like as powerful and well organised as Caesar insisted them to be, they should have featured constantly in those wars, yet they never appear in them at all. Many modern authors, therefore, have charged him with exaggerating the importance and organisation of the Gallic Druids. By doing so he made the Gauls seem more dangerous and more worthy as adversaries and so his own conquest more glorious.
In general, Greek and Roman accounts of Druids fall into three categories. Some, mostly Greek, treat them as great philosophers and scientists worthy of admiration. Others, mostly Roman, make them into bloodthirsty barbarian priests, epitomes of backwardness, ignorance and cruelty. Yet others, like Caesar, suggest that they were both. We have no means of telling which are closest to the truth. In general, the further away from real Druids an ancient author lived the nicer he tended to think they were. This could mean that the more favourable accounts of them are mere wish-fulfilment, fashioning romanticised portraits of noble savages. Those who lived closer to Druids may be regarded as staying more faithful to a brutal reality. On the other hand, the writers who were geographically closer to Druids had the strongest possible motive for exaggerating the danger and the horror that Druidry represented, justifying their conquest by Rome. By this reckoning, the more favourable accounts, mostly produced by Greeks who had themselves been conquered by Rome, could be the more truthful. We can never know.
The second group of sources consists of portions of medieval Irish literature. These have the virtue of being produced by a society which itself had once included Druids. Furthermore, the references to Druids in Irish stories are far more frequent than those in Greek and Roman sources. There are, however, two problems with the Irish texts. The first, which they have in common with those from Greece and Rome, is that some portray Druids sympathetically as figures of great wisdom and power and some represent them as savage pagan priests. The second problem is that all the Irish texts were written, and perhaps composed, hundreds of years after the conversion of the Irish to Christianity when Druids had by definition ceased to exist. As the Irish had no writing before they became Christian they would have been dependent on oral tradition for information on their pagan past and we have no idea of how accurate that was. The findings of archaeology are not encouraging in this regard. The sites identified in the tales as important in the Iron Age were certainly occupied during that period. However, they were open ceremonial structures and not the great royal banqueting halls portrayed in the later stories; the composers of the latter may have been inspired by the sight of ancient ruins on which they had no real information. …