Ireland come to still hold to pagan myths from their distant past, blend them into the warped thinking that a carved turnip (later Pumpkin) can be used for scaring away spirits?
I'm guessing those spirits aren't afraid of a carved squash of any kind...just saying... For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places. Ephesians 6:12
Q: Where were the shepherds of the flock all those centuries this was forming?
A: Oh, that's right...there were teaching massive errors on "hell", state of the dead and so-called purgatory.
"The tradition of carving Jack-o’-Lanterns
originated in Ireland using 
turnips instead of pumpkins.
It is
allegedly based on a legend about a man named Stingy Jack who repeatedly
trapped the Devil and only let him go on the condition that Jack would
never go to Hell.
But when Jack died, he learned that Heaven did not
want his soul either, so he was forced to wander the Earth as a ghost
for eternity. The Devil gave Jack a burning lump of coal in a carved-out
turnip to light his way.
Locals eventually began carving scary faces
into their own turnips to frighten away evil spirits." HeatherThomas
