One of the ideas that druidic/ancient Celtic belief seems to have held dear was that of reincarnation. It’s a bit hard to have purgatory or an afterlife as we think of it in Western traditions if you also have the idea of reincarnation. There’s not much point is there? There were no druids in Germanic areas according to the ancient sources – the Romans noted this as being a major difference between the Gauls and the Germans.
Pope Gregory IV transferred the celebration of All Martyrs from May 13th to November 1st in 835 CE. May 13th had been the date of the Roman festival of Lemuria, when spirits and ghosts were banished from a Roman household. In the early 7th century CE, Pope Boniface IV used this date, May 13th, to rededicate the Pantheon in Rome to Saint Mary and All Martyrs. As far as Samhain is concerned, there’s not much to link it to the dead and the ancient Irish festival seems to have been more connected with feasting. Furthermore, Samhain seems to have been specifically Gaelic and there’s nothing to suggest it was all that important elsewhere or some kind of pan-Celtic festival. Even the supposed link to Samonios on the Coligny Calendar is debatable. Interestingly enough, entirely outside the Celtic sphere, so to speak, but well within the Roman one, in various parts of mainland Italy, Sardinia and Corsica, Hallowe’en-like festivals connected to notions of spirits, souls and the dead are celebrated as part of the traditional folklore calendar.
Now, I like Hallowe’en and I enjoy the festival with all of its influences as it has come down to us today, but it’s not particularly pagan nor particularly druidic and an awful lot of very spurious claims have been made about it. I really don’t understand why many feel the need to try to claim origins for something in order either to attack the one tradition or seek to debunk the other.
As for karma, well, it’s a Sanskrit word that basically means action or deed, so tracing back the Indo-European roots we’d get Proto-Celtic *kwert which gives us the Welsh word peri, which means to cause or induce; however, I do not believe the Welsh word holds the same spiritual meaning.