In Ireland, Halloween is called Oiche Samhna. The origins are based on an old Celtic festival of Samhain. Samhain also means November in Irish. In old Celtic Ireland the year was divided in two – between the lighter half (summer) and the darker half (winter). During the eve of Samhna it was believed that the division between this world and the other-world was at its thinnest, thereby allowing spirits to pass through from the other-world to this world. During the festival people would welcome their own ancestors into their homes but would try and ward off unwanted spirits by disguising themselves with masks and costumes.
Wishing you all a Happy Samhain