Druids were the ancient religious leaders, mystics, philosophers and tribal judges of the Celts in Britain, Ireland and Gaul (major part of modern day Western Europe). Much of the information we have about druids comes from roman times when the Romans were expanding their empire towards the west. References to the druids were also made by ancient Greek writers and during the middle ages.
There are a number of points that seem to connect druids with the ancient Greek civilization. Druids get their name from the Greek word for oak tree which is “driis” (in Greek ΔΡΥΣ). Since time immemorial, the oak tree was a sacred tree for ancient Greeks. The oak tree was the sacred tree of the Greek royal families of Macedonia. Many of the Macedonian crowns or wreaths found in royal tombs in Greece depict oak branches and leafs. The oak tree was also the sacred tree of ancient Greek gods Gaia and later Zeus. Also, an oak tree believed to have prophetic powers stood by the ancient oracle of Dodoni.
In the Gaellic Wars a book written in Roman times by Julius Caesar, we can find evidence that druids used the Greek language:
” They (the druids) are said there to learn by heart a great number of verses; accordingly some remain in the course of training twenty years. Nor do they regard it lawful to commit these to writing, though in almost all other matters, in their public and private transactions, they use Greek characters. That practice they seem to me to have adopted for two reasons; because they neither desire their doctrines to be divulged among the mass of the people, nor those who learn, to devote themselves the less to the efforts of memory, relying on writing; since it generally occurs to most men, that, in their dependence on writing, they relax their diligence in learning thoroughly, and their employment of the memory.”
Lebor Gabála Érenn (an ancient Irish collection of poems and narratives) refers to the Greek Partholon who reached Ireland after the big flood. He came from Macedonia or central Greece with his wife; his three sons and their wives and three Druids, Fios, Eólas, Fochmarc. The names of the three druids mean cognition, κnowledge and inquiry. As it happens with myths they carry a core of truth. In this case the story could signify that knowledge was brought to Ireland by Greek settlers who brought with them wise men to spread knowledge and educate locals (druids).