Besides the story of the Irishman who tried to trick the Devil, I recently found this somewhat more down to Earth meaning of the Jack o Lantern;
"Jack-o'-lantern" first referred to night watchmen who carried lanterns because there were no street lamps.
Jack, a popular diminutive of the name John, was often used to mean "man," and as a result, a man carrying a lantern was often referred to as a jack-o'-lantern.
When turnips, carved with faces to ward off evil spirits, were lit from within, they were considered lanterns, too. Eventually, the term "jack-o'-lantern" came to refer to the smiling pumpkins we associate with Halloween.
It must have been some job, carving a turnip!