A leprechaun was a magical creature in Irish folklore.
After Tom Paris created the holoprogram Paris 042, set in Fair Haven, Ireland, Harry Kim remarked that the only thing missing were the leprechauns. In emphasizing the holoprogram's authenticity, Paris replied, "no leprechauns, no aliens, no starships." (VOY: "Fair Haven")
Later, Maggie O'Halloran noted the similarities between Neelix and a leprechaun. (VOY: "Spirit Folk")