ahh - druids (will Atlas ever dare the venture of a druid/diedne book...?)!!
As Ravenscroft also said our knowledge of 'the' druids is quite limited. Their tradition(s) mainly oral and/or perished the main sources are from a biased Roman and/or Christian world view.
I got myself some books on druids some years back as I expected our ongoing saga to turn in that direction - giving the title will probably do you no good as they are in Danish 
In short I'd recommend the long range of Roman writers who commented on the druids. Not because they are less biased than later writers but because I their bias is quite colourful and often the stuff interesting plots are born from. E.g. did Pythagoras teach the druids or is it vice versa?
It is interesting how many of the later concepts of for example witches, even the modern stereotype picture, in fact resonates with several features of the druids. Picking them apart some of the original source for those stereotypes might be found.
Some short aspects that might be used to set the theme:
Beliefs that are foreign to standard ME cosmology such as a sort of reincarnation - not only as new human beings (could be in the same family line) but as a tree, a puddle, a fly etc. These cycles are often linked directly. One 'state of being' being swallow by the next or born from the previous. These are not necessarily 'soul journeys' as much as a change in substance. Muto? - and the sort of kind not limited by Essential Nature? In essence being a girl is not essential different from being a puddle.
Weather magic. Julius Caesar had trouble with his men during one of his landings on Anglesey (the Unnamed Covenant ehh?). They were assailed by thunder and lightning - and his men lost morale and Caesar had to convince them of the truth of the matter; that what they were fighting - what lay in store for them on the beaches was only old men and women not the measure of the mighty storms they might raise from afar. In fact even if Caesar had his ventures concerning Britain I am suddenly in doubt whether he was the leader of said landing on Anglesey..?
The cauldron. Cauldrons were used for all kinds of ritualised magic and magnificent works of art were done on those, and some remain to this day. Besides being used for all sorts of things (one god was said to have a caldron that could feed any gathering) - one in particular seems widespread: resurrection cauldrons. These cauldrons could resurrect dead warriors after a battle - which led Arthur in some Welsh stories to on a quest for a cauldron on faraway island (who said the Monthy's were all original?). Not entirely unfittingly several of the cauldron in museums today has been found in various bodies of water - often split in parts, as a sacrifice or maybe to ensure that enemies didn’t retrieve the resurrection cauldron (warring kings had a fuss plotting on stealing or protecting such cauldrons). Many Celts - as well as Thracians - practices ceremonial drinking from a cauldron prior to battle. This was to ensure an 'inauguration' to the battle so that if they fell on the field they would be resurrected in the afterlife - often to continue an existence of battle, valour and feasting! This tradition particularly resonates with the Norse concept of afterlife in Valhalla, as well as some aspects resonate with the 'promise' of Christian Eucharist. In terms of the resurrection cauldrons and ME you might decide that druidic magic is to some degree not limited in bringing back the dead, in some circumstances at least, but then you'd have to decide whether that limit is Hermetic only or whether it is by Divine ordinance.
Animals. A certain level of respect toward animals might also add to the mood, at times bordering on reverence. Not necessarily toward all animals, but often toward a certain kind. Just ask this forum's "Cuchulainshound"... This not only in how you treat animals but also in terms of the power of names - transgress against a certain sort of animal and you also dishonour those that share their name.
Plants. Certain plants had specific significance; Most of all the mistletoe - or 'uil Ãoc' (allcuring) in modern Irish. It's significance might be due to the general reverence of oaks and because it might grow on them. Secondly the druids seem to have been very focused on categories and that being in-between the categories are at one time 'dangerous' and an importunate opening to a world beyond. The mistletoe neither grows on earth nor flies so it is an example of not categorise-able.
Women. Giving the female a different position than in the typical ME might be interesting as well - as a reminiscent of ancient fertility cults the female had a more equal counterbalance to the male. When I read Ancient Magic a few weeks back my first thought was to include some of the feel and/or mechanics of the Fertility chapter.
Sacrifices. Works of art, food, beverages, remains of the dead or sacrifice of the living... And the druids had a certain role in these sacrifices, even if it is unclear. I recall that both Timothy Ferguson and The Great Sun Jester have mentioned the Wicker Man at earlier threads (burning people alive in large.. well.. .wicker men) and you might also draw inspiration from the movie of the same name - at least on how to scare your troupe! 
The question of human sacrifice however is also closely connected to hermetic history and the Schism War so this might not be usable depending on your take on the Diedne.
Cycles. The cycles of the day and the seasons are of great importance. Even if much older than the druids the monolithic 'clocks' dotted around old Gaelic lands continue to have importance. Solstices and equinoxes are of great importance. This might very well include a waning and waxing of their magical might (m-might as opposed to M-might). Feasts were an important factor of the cycles as well.
Location (I was tempted to write Correspondance..): Just as with time druid/Celts seems to have put great significance on location - centre and periphery; Perceiving one self in relation to the direction and distance to certain fixed points. This includes a real sense of the boundaries between the world and the otherworld. You might give them a natural proficiency in navigating Regios - maybe even creating them. Together with the cycles of time this parameters might let them achieve truly magnificent results.
Bards. Their tradition is not only oral - it is about memory, poetry, verses and music as well. The poets were in ways organised and also closely connected to the kings. Even the threat of creation of spiting verses was presumably a check on the power of the king and potentates.
Clarity of vision. In Irish writings, as opposed to the Roman ones, the druids are not so much characterised by wisdom as such. Instead they are told to have a certain 'clarity of vision' - that they are not only hard to deceive but that they have a power of perception beyond normal - both in terms of the senses and in terms of the 'thought processes' of taking in the impressions. In turn Cicero (and Caesar too) both talk of druids with the power to foretell the future - which might also be a consequence of the clarity as well as the spatial and temporal awareness.
Pheew - couldn’t stop! 
In short I'd recommend you to get inspiration from the Roman authors and to read a few Celtic parables - they both give amble inspiration for backdrop mood as well as stories.
I'd make them very inept at a range of Techniques/Forms (if even describing them in those terms), but make them formidable in a few others. I'd let them break the Limits on some areas and I'd make some flaw/virtues a natural part of all their powers.
[EDITED for too many typos and bad wording]
) but we do not have much iddea on what areas of specialization and powers they shoulod have.
