Be careful attributing many Hallowe’en customs automatically to supposed ancient “druidic/Celtic” customs or Samhain. A case in point, trick or treating or “guising” as it was also known, may have originated when poor children would go from house to house asking for food and sweetmeats in exchange for going to a church and lighting candles and praying for the souls of one’s ancestors in purgatory, and in pre-Reformation Britain people would exchange “soul cakes”.
In fact, a lot of the associations with death and souls are actually Christian ones, hence All Hallows Eve, All Saints Day and All Souls Day. If anything, the associations with the dead may rest more upon non-Christian Roman traditions that were Christianised centuries before. Subsequently, the date was moved to October 31st in the Middle Ages, however, seeing as this was a date that was chosen to standardise the festival throughout the Christian world of the time, claims that it was deliberately chosen because of a relatively obscure Irish/Gaelic custom in one part of the world don’t really hold water. With the Reformation, a lot of traditions that were actually Catholic traditions were denigrated as pagan, diabolical and so on and relegated, when not eradicated completely, to rural folklore and customs.
“Halloween is probably the most misrepresented and misunderstood festival in the traditional calendar. The widespread notion that the day (or rather the night) is a pre-Christian pagan celebration is not historically correct, but is now so well-entrenched as to be immovable.”
Entry: Hallowe’en (31 October), p.230 in The Penguin Guide to the Superstitions of Britain and Ireland, Steve Roud, Penguin Books (ed. 2006)
As for snakes/serpents and, by extension, dragons, there are also a couple of problems. First and foremost, there have not been an snakes in Ireland since at least the Ice Age and when “serpents” do appear in Celtic mythologies, they rarely appear in a positive light, one example that comes to mind is the battling of the Fianna against dragons/serpents throughout Ireland. In addition to this, with probably the exception being the Gundestrup Cauldron and the odd representation of what might be Cernunnos, serpents/dragons do not feature very much, if at all, in pre-Christian Celtic art. The stories of snakes, serpents and serpent-like water beasts, e.g. Morag and the Loch Ness monster and so on, seem to originate in the early Christian period with the legends surrounding Saint Patrick, Saint Columba and Saint Petroc. As stated before, they are conspicuous by their absence in Gaelic traditions and their appearance in Brythonic traditions may be more due to Roman/Mediterranean influences.