I've been taking a crack at an Infernal version of the Diedne recently.
In my approach, the druidic traditions of Gaul were driven fully extinct; weakened by Roman suppression of the druids, diluted in Gallo-Roman pagan syncretism and the decline of Gaulish Celtic in favor of Latin/French, pressured by Christianization, and finally extinguished in the barbarian invasions. By the time of the foundation of the Order, the hedge magic and remaining pagans of Gaul had nothing to do with druidism.
However, the insular Celts retained something more. As suggested by The Contested Isle, p.112, Hibernian druids and the Caledonian gruagachan (from Hedge Magic, p.55) were branches of the same tradition, a tradition that also had a (south) British branch. That last branch was weakened more than the other two because of the Roman conquest, but survived better than the one in Gaul into the post-Roman period. This tradition was more skilled in spontaneous magic (Cailleach Magic-equivalent, being derived from an earth goddess part of the tradition) than the Hibernian or (standard) Caeldonian branches.
Then, under the pressure of the Anglo-Saxon invasions of Britain, when a number of the British fled Britain and settled in Brittany, the followers of the tradition went along. Here they wound up face-to-face with local Roman-derived traditions, which (unlike the gruagachan-like traditions) had powerful ritual magics available. The druids of Brittany accordingly felt a lack, and went to fill it -- based on the available Roman accounts of the druids.
So, whether the "real" pre-Roman druids did human sacrifice or if that was a Roman slander, the new Brittany druids thought they were supposed to, and did . . . and beings claiming to be Esus, Taranis, and Teutates answered and empowered them. These were demons, who taught the druids of Brittany something similar to a Mystery Initiation that gave, from RoP:The Infernal, the Favored abilities of (Infernal) Ceremony, Effusion, Incantation, and Malediction to people in addition to their existing abilities. The result was a "Misguided Tradition", where the Brittany druids thought they were practicing pagan rites, but were actually serving Hell.
Now, note RoP:Infernals has a specific section -- p.95 -- on how to empower Infernal abilities with sacrifices, in addition to the power that can be added by Ceremony.
So, when Diedne joined the Order, she brought along her tradition's secret "pagan rites" with her -- as seen when they stripped the Picts of their language with a level 55 Incantation/Effusion to give every speaker of Pictish on the island of Great Britain a -6 to their ability in that Living Language with the Forsaken duration (the event described in Hedge Magic, pp.72-73, using the guidelines on RoP:The Infernal, p.109). With the knowledge that the rites could be empowered by sacrifices, especially of humans. But while Diedne was merely misguided, her followers eventually delved deeper into these "mysteries", developing initiations into other Infernal abilities. By the Schism War, the House was led by those who had "completed the threefold initiation" by two more initiations to add Debauchery (useful in calling up storms and the like) and Diablerie.
And then, caught and warred upon, the inner circle fled far to the east, and hid among the Mengwu Shiwei. They've been building up their numbers in exile, creating their own apprentices through Diablerie to instill the False Gift as necessary . . . and now have begun an effort at world conquest with the Mengwu Shiwei -- or "Mongols" -- as their pawns.