The information we have to work with for understanding the Druids falls into four categories Greek and Roman writers, archaeology, comparative evidence, and medieval Irish stories.
Greek and Roman writers
Greek and Roman writers from antiquity writing about Gaul and Britain.
The study of human remains found in graves can help us understand something about the people themselves, such as how large they were in general, how long they generally lived, the rates of infant mortality, and sometimes even the illnesses that plagued them. Archaeologists can also help us understand a little of how they worshiped, where they conducted ceremonies, and the objects they offered.
Indo-European culture Comparison
Evidence of how words, especially the Indo-European roots from which they originated, evolved in the Celtic languages. In certain cases, the most speculative evidence we have to work with is linguistic material, as it always depends on hypothetical reconstructions of past actions for which little or no independent evidence exists. To see what similar systems and organizations they have, we may also look at other Indo-European cultures. This, however, may also contribute to erroneous assumptions: even though cultures derive from the same people, they frequently evolve in distinctly different ways, forming various social institutions, religious beliefs and practices, and cultural norms. In other words, the fact that Hindu Brahmins can be proved to be true by an action or belief does not mean that Celtic Druids are similarly true.
Gaelic medieval Christian monks
Irish culture had been overwhelmingly Christian by the start of the seventh century. The earliest written proof for Ireland dates back to the moment when Irish monastic scribes began to keep records of The lives of patron saints, notes of events, laws, and some other common lore inside and outside their monasteries.As time passed, additional material was documented by the scribes, including fragments of pre-Christian myths and what could be considered “secular” tales about past heroes. Druids are listed in some of these materials, but the explanations interpret them from an often negative Christian viewpoint. Even the earliest legal material presents druids as a group whose reputation has become questionable in society: Druids are portrayed as agents of demonic powers in some stories, or they are portrayed as arrogant and untrustworthy. Even if the pictures of druids are reasonably even-handed, we can conclude that the authors and storytellers knew nothing about what the druids did or believed, so that their explanations are unlikely to be entirely precise.