Druidry has probably had more bullshit written about it than anything else. Druids were a Priestly caste of people, contiguous with, but not ethnically related to Neolithic Europeans. Their influence was throughout Western Europe, and Scandinavia. It's roots go back certainly to Mesopotamia, predating the Bronze Age, as can be seen from the Astronomical Layout of Stone Circles. From a purely practical point of view, it was a big help to the stone people age to have some geeks around who could build accurate calenders, (and read them) It enabled the first Agriculture to happen. Their Astronomical influence can be seen from the Himalayas to the Baltic, from the Med, to Scandinavia, from the Baltic to Egypt, and Western Africa. Everywhere, in fact that the Phoenician trade routes went.
About 4-5000 years ago there seems to have been some kind of diaspora from the Mediterranean & Levant, possibly due to Volcanic Activity, coinciding with the spread of Bronze, Metallurgy, the first Warfare and the appearance of religious traditions that weren't based solely around Corn Goddesses.
This is when the people we think of as "Druids" consolidated their position into a Socio-political network of educated "Priests" and were evidently centered, and trained for this Priesthood, from an ethnically Brythonic Britain & Ireland. (Remember this is still a long time before the Fathers of the Celts came North) This is borne out by Lingual influence. The only traces of Brythonic language are Welsh, & Basque (the Pyrenese being another cultural outpost, like Britain) with the same genetic marker as the fair skinned people found (and butchered) in the Canary Islands, by the Spanish in the early 1500s.
Brythonic is totally unrelated to any other Indo-European Language, and no-one can say for sure where it originated, but it's probably as old as the Beaker-folk. Who were pre-agricultural Paleolithic from when Britain still had a land bridge to Northern France. This is borne out by a sudden industrial regression in Britain, coinciding with the end of the last vestiges of last Ice Age. The art of making earthenware wasn't picked up again for thousands of years in isolated Britain. The legendary first King of Albion was Brutus, who was supposed to have arrived in Britain as leader of a diaspora of Trojans, fleeing the wrath of the Atreides after losing the Trojan War.
They settled in the area where London is now. London was established as a trade post by the Phoenicians, being the farthest inland they could sail their longships up the Thames.
The earliest buildings there, were Temples to the Gods of Arcadian Greece. And when the Romans began trading there, the predominant Tribe in the area were the Trinovantes (Troy Novantum) or "New Trojans".
All this gives a period of anything up to 4000 years for those early Chaldean Priests, (themselves driven out of Babylon by the spread of the Jehovah Cult) to establish socio-political ties with all the Tribal civilisations of Western Europe and Scandinavia. Also why the Romans were so keen to wipe them out. As the Roman Empire stretched across Europe, these "Druids" had a perfect resistance network set-up.
In fact, technically, it was only last year that the Roman Law "No Druid shall be allowed to set foot in Britain, upon pain of death" was lifted, with the State acceptance of "Druidry" as a religion.
What we classically think of now as a Druid, is mostly based on Victorian romantic revivalism, and has very little basis in fact.
The only written historical reference to Druids was by Julius Caesar (The Romans, were admittedly, biased) who painted them as Barbaric, Human Sacrificing Madmen, with stories of people (Romans, mostly) being burned alive in Wickermen. Just standard Romanist propaganda. The archaelogical and cultural markers though, can be traced much more surely than Caesar's sparse mentioning of "Druids". So the whole "Druid" tradition as we think of it today, was just the Victorians romanticizing the tail end of a far older, nameless tradition that spans the whole of History, encompasses the whole of civilization, and goes back to the very first Astronomical cultures which we can trace back to the Stone Age.
They were probably the original Dionysiac Architects or "first builders" referred to in Masonic lore, (Itself partly lifted from the earlier Orphic Mysteries) They're possibly descended from the Nephilum mentioned in the Old Testament.
Definitely, the Culdees, who were mercilessly put down as Heretics by the early Catholics, were Chaldeans. ("Culdees" = Chuldees or"Chaldeans") Also, the Serpent Cults, that were always the "Bringers of wisdom" may well have been a reference to the Serpent prowed longboats of the Phoenicians, (Made famous by the Vikings, whose Fathers were descended from the Phoenician Sea Traders) who carried the diaspora of Chaldeans to the far flung shores of Britain. (Possibly even as far as Mexico too.)