There's some doubt as to whether these were just the four main ones which had law court sessions attached to them too, and the other festivals weren't mentioned by early christian chroniclers as being vulgar pagan folk festivals.
I like Ellis-Davidson, got a few titles mostly relating to northern tradtions though. MacNeill, Máire: The Festival of Lughnasa (Oxford University Press) 1962. Republished 2008. ISBN 0-906426-10-3. It's the book all the other books used to quote, but was very rare to find a copy. It's 900 pages, but I heard the new print might be in two books and somewhat abbreviated.
Bull's Blood
"Paganism took a very long time to die out in remote parts of the Highlands and Islands. In connection with the Celtic festival of Lughnasa, at around the end of August bulls were sacrificed on an island in Loch Maree. This feast had a formal association with St. Maelrubha, an early missionary, but his name had been clearly substituted for a pagan deity. In the 17th century the presbytery of Dingwall was outraged at the persistence of this: 'amongst other abhominable and heathensiche practices that the people in that place were accustomed to sacrifice bulls at a certaine tyme on the 25th of August, which day is dedicate, as they conceive, to St Mourie as they call him'. In much earlier times Druids supervised the sacrifice of bulls both in pursuit of divination and also for votive purposes, often on behalf of a king. Perhaps the bulls of Loch Maree were sacrificed for the kings of the Caereni, the Sheep Folk."
Some would say that the Pagans and Druids are still on the go................