The word that became anglicised as druid comes through Roman and Greek records of their interactions with their European neighbours, who they viewed with the same murky light that Europeans went on to view indigenous folk across the globe. Nevertheless, by modern standards, we would probably view the Iron Age religions in the same light given their tendency to human sacrifice – having not yet made the leap to animals as a proxy as the Greeks and Romans had or the symbolic use of food and drink as the later Christians did.
Druids were usually discussed in plural in these early records and there is a certain assumption of a semi-organised and systematic approach to their religion. Traditional assumptions certainly tend agree with this, with the Paulinus’ attack on Anglesey (Ynys Môn) in 60 AD usually being described as the final step in destroying the druidic religion. Anglesey was apparently considered the centre of the “cult” but those who follow the Celtic Interpretation of Blessed Isles and Otherworld histories suggest that this may have been down to the confusion in the name (Manannán mac Lir) and the real birth of druidism may have been at Emain Ablach (the Isle of Apples) or some other location referred to with some variant of Man (Mountain).
In Welsh traditions, the power of druids was essentially wiped out with the associated word (drwy) potentially a borrowing back from Irish and used to cover the “seer” element of poetry and magic. In Irish traditions, druids appear to have retained knowledge and power for some time – until Christianity became the major religious power in the 7th and 8th Centuries – but the records do not seem to suggest an organised, centralised approach continued, if it indeed existed, following the attack on Anglesey.
There were revivals of Celtic and druidic traditions based on interpretation of folklore and written record in the 17th and 18th Centuries, which will have fed into the Romanticism movement of the same periods. However, existing neo-pagan druidic groups are a 20th Century revival. That said, this has not prevented their rites and worship from strengthening the connection with the Otherworld at a number of significant Waypoints, the most obvious example being Stonehenge.