Before Christianity hit the Emerald Isle, the Druids celebrated Samhain (pronounced SOW-wen), or the festival of the dead. Samhain was held at summer's end when, the Druids believed, there was but a mere veil separating the worlds of the living and the dead.
Samhain involved a communal feast, setting out food for the dead and leaving doors and windows unlocked so the spirits could travel freely. The Druids also believed evil spirits would try to cart off unsuspecting souls at this time of year, so they would don costumes mimicking those of the spirit world in order to protect themselves from being taken — the predecessor to today's Halloween costumes.
When Christianity came to Ireland, the Catholic Church replaced Samhain with All Saints' Day and All Souls' Day to provide a "better" celebration than the Druids' pagan ritual. Even so, many of the old customs remained. People enjoyed parties, apple-bobbing and various divination games. One example of the latter: peeling an apple into one long strip, then tossing the strip of apple peel over your left shoulder. The strip will supposedly fall into the shape of the initial of your future spouse. Partygoers ate special foods, like barmbrack, a type of fruitcake into which various small items were baked. The item in your slice of cake denoted a future occurrence. For example, getting a coin equaled prosperity, while a ring meant romance and/or happiness. Ancient Celts also carved turnips, a precursor to the American jack-o'-lantern [sources: Haggerty, Ireland-Information].
Today Irish kids still dress up for Halloween, go trick-or-treating (now for candy instead of the traditional apples and nuts) and eat barmbrack, among other customs linked to the Druids. Some modern-day Druids have revived the ancient Samhain celebration as well.