This is an interest of mine that I’m quite enthusiastic about. However, so much of the information available was either through contemporary sources who had a political agenda, compiled way afterwords, filtered through the not-very-historical lens of neopaganism, etc. For that reason, I hoped that the people in this sub might be able to help me find more reliable history.
Anyone who could point me towards sources for further reading would be greatly appreciated! Any direct thoughts, knowledge or experience you have on the topic would also be amazing. Thank you so much for your help!!
Edit to add: also happy to hear about folk magic which did incorporate Christian traditions and elements, I just didn’t want to make the question too broad! As long as it’s people trying to do magic, it’s hella cool.
While waiting for a real answer, you may enjoy these links
Can you give me an overview of magic in medieval Norse society? with /u/JDHoare giving a lot of information.
(H)ow much is actually known about the druids? has a lengthy list of other posts at the top.
Sources on female druids has /u/Kelpie-Cat giving some neat info and name dropping a book you may be interested in reading - The Blood and the Mistletoe. Here's a brief description of the book
And branching a bit out from what you were asking - When did the depiction of Wizards using magical staffs begin in European Fiction, and did it's origins trace to European cultural history or a non-European cultural tradition? which discusses the ancestry of the archetypical "Old man with a staff" wizard, though you probably want to jump ahead to /u/epicyclorama 's comments.
Here are a couple of sources covering Scotland.
The Picts by Benjamin Hudson has a chapter on spirituality, and a brief section on pre-Christian beliefs. Amongst other things he discusses Broichan, a magus in the court of King Brude:
Magi were believed to have control over the elements and it is implied that they conjured up contrary winds to delay the saint [that is Columba] and his companions on their return to Iona.
Alcock's Kings and Warriors, Craftsmen and Priests has a short chapter on pre-Christian beliefs, mostly about burial practices. It can be downloaded for free from here:
https://archaeologydataservice.ac.uk/archives/view/sas_books_2016/bookcontents.cfm?mono=1369054
You can also look at Joyce Miller, 'Beliefs, Religions, Fears and Neuroses' in A History of Everyday Life in Scotland, 1600 to 1800. Miller discusses magic and popular religious practices, some which was an amalgamation of Christian and pre-Christian practices:
The use of the number three as a representation of theTrinity was widespread. Individuals often walked three times around a holy well, a tree, a house or some other site which was believed to possess special powers, with the afflicted or with the suspected cause of the problem, possibly someone suspected of using witchcraft. Clearly the number three has recognisable Christian overtones, but it also has older origins as tripartite divisions were a fundamental aspect of Indo-European culture and society, and three-fold patterns or shapes were recorded in pre-Christian Pictish symbols and stones.
The Peat-fire Flame: folk-tales and traditions of the Highlands & Islands by Alasdair MacGregor, contains numerous tales with magic and spells and fairies. The book can be downloaded from the Internet Archive:
https://archive.org/details/peatfireflamefol00macg
If you’re interested in the role of animals in ritual, Miranda Green’s Animals in Celtic Life and Myth might be a good source. She discusses animal sacrifice, ritual feasting, burial practices, sacred animals,and the different animals represented in ritual. According to her:
Ritual behaviour involving the deliberate killing of animals was endemic in Celtic society. (…) Sometimes the animals involved in Celtic ritual were eaten or partially eaten; sometimes whole or parts of bodies of beasts were offered to the gods. It is possible to observe a complex but systematic behaviour-pattern in animal ritual. (…) It is crucial to recognize that for the Celts ritual behaviour was not marginal but central to everyday life. This is why such behaviour manifests itself not just in sacred places nor within the context of funerary activity, but in the places where people lived their daily lives.
There is a chapter on Celtic religion in The Celtic World (edited by Miranda Green) which might be of interest to you. It contains the following articles: