Some of the Atlanteans managed to escape and set up small settlements in obscure places along the European and African coast. As they came from an island, or an archipelago of islands, the Atlantean refugees tended to settle on remote islands offshore. Some settled in the Hebridean Islands off Britain and on the islands off the Irish Coast. One of these settlements was that of a group of sorcerers who encountered the indigenous shamans and set off a process which resulted in the Druids.
The shamans of Britain and Ireland at that time were hunter-gatherer shamans and at first thought the Atlantean Sorcerers to be Gods. After many years, they realised that they were merely learned men and began to learn from them and to teach them their secrets. Over the course of many years, the shamans and sorcerers became closer and closer until they merged to form a new order of priests. The original Altanteans did not themselves change, but their descendants did, becoming the first Druids.
The Druids succeeded because they were a class apart from normal Priests. From the Atlanteans they learned how to be Priests. From the Shamans they learned that all gods were essentially the same and that you could worship more than one god using the same techniques. They managed to fuse together these beliefs into one system of worship that was Pantheon rather then deity focussed. When the Celts came across Europe and settled Ireland and Britain, the Druids found a new Pantheon that they could worship and develop, so they became entwined in the Celtic way.
As the Celts held Islands sacred and thought of the Atlanteans as Fairy Folk they held the Druids in awe as they were descended from the Fairy Folk and lived on Islands. It became easy for the Druids to become accepted and to become the leaders in the Celtic religion. Later they encountered the Roman and Christian Pantheons and became integrated in both, to lesser degrees.
Since Druids descended from both Shamans and Sorcerers, they included both mind sets in their practices. Thus, they could manipulate Spirit Magic in the same way that sorcerers manipulate their own spells. Treat this as RuneQuest Lunar Magic (Amplify, Prolong, Combine etc) but without the Lunar Cycles. This makes Druids powerful as they can cast high-intensity spells and also have access to more spirit magic than normal folk.
Druids also practised Divination. This can either be the normal RuneQuest Divination spell, but taken from individual deities, or the Druids can have a Ritual Skill of Divinate which allows them to see into the future, have knowledge of signs and portents or do the other things that Druids are supposed to have done.
When the Druid stronghold on Anglesey was destroyed by Julius Caesar, the Druids went to other places and set up their own schools. Ireland became a focal point, as did the mountains of Wales and Caledonia, the marshes of Devon and Cornwall were also important spots together with Exmoor and Datmoor. Some Druids also settled on the Isle of Man and Holy Isle. Thus, the druids were scattered throughout Britain and Ireland, never again to be a united, focussed threat to the Roman Empire.
From the ancient Irish and Welsh texts, each king had one or more druid attached to his court and these druids would help the king become more powerful, casting magic to assist him and defeat his enemies. They were completely involved in politics. Look at Merlin - he was a meddler in politics from start to finish.
There are accounts of druids who were the sons of druids, so they must not have been ascetic individuals.
There may have been female druids - Julius Caesar recounts fighting male and female warriors on Angelsey. I can see no reason at all why not to have female Druids.
On Angelsey, the druids fought with incredible ferocity, so they were able warriors. I see them as having a warrior’s training as well as a priest’s one, enabling them to understand the warriors and able to lead warbands if necessary.
Druids were also skilled healers, being masters of healing plants and able to access the Healing magics of the deities in their pantheon. Druids who knew of Bran could even resurrect the dead - making them fearsome magicians.
This combination of warrior/healer/priest/sorcerer made the Druid someone to be honoured and feared by the Celtic peoples.