Nothing was substantiated by the article.
The fact of the matter is that we know exceedingly little about Celtic Paganism in general. Of our written sources we are left with Greek and Roman sources (as I mentioned earlier, we have Julius Caesar's account about the Gauls), but these are often contradictory. I mentioned the druids previously, who seem to be whatever the author wants them to be. Our only other written sources come from medieval Irish monks; and it is hardly clear what in these Irish fairy-tales, written centuries after the conversion of the Irish and other Celtic peoples to Christianity, is genuinely representative of pre-Christian Celtic pagan tradition, and which is the product of later Christian imagination.
The few things we can seem to peel away and get at is that there were four major seasonal celebrations, one of which being Samhain, the Celtic start of the winter season. Legends, myths, and fairy-tales from Irish monks and other Christian writers tend to indicate that there was a belief that during Samhain the barrier that separated our world from the "other" world--the world of fairies, grew thin. And so fairies and spirits could cross over.
The problem, of course, is that connecting that with Pope Gregory IV promoting the celebration of All Saints on November 1st among the Franks, as it had become the custom among the Romans is, basically, not possible.
-CryptoLutheran