It's strongly a Christian celebration, and goes back to at least 313AD. The issue is on what date it's kept.

The popular conception is that it's Celtic in origin and the Church christianized a pagan celebration. It is true that Samhain is celebrated on the 1st of November, and it's a celebration opening Winter, but it was two English historians who introduced the notion that Samhain was the Celtic New Year and a feast for the dead, (John Rhys and James Frazer) but they way in which they did this was somewhat arbitrary. Rhys simply took much later folklore and extrapolated back, rather than having any early documents at his disposal. Frazer simply assumed that since there was a Christian festival, then they must have syncretized with a previous existing pagan festival. There isn't any evidence for either assertion. The other problem is that the celebrations were ultimately Germanic in origin, not Celtic, as the Irish kept the 20th of April as All Saints, so there goes the association with the Celtic new year.

What we can say is that at least in popular folklore, the festival was associated with supernatural forces (in the same way the opening of summer is associated with fairies abroad), but it's not connected to the death or new year. The problem with folklore is that a lot of what is recorded is from the 16th century onwards - what we're asked to believe that these are accurate (or reasonably accurate) accounts of what took place prior to Christianization of Ireland, circa 5-6th centuries. That's a gap of a good 1000 years, so it's pushing the boundaries.