A leprechaun is a small green magic person from Ireland who guards a pot of gold at the other side of a rainbow. Notable leprechauns include Saddam Hussein (pot of gold = WMDs), the Pope (tiny green man), Napoleon(fairly obvious, he's magic), etc. Everyone knows leprechauns exist as there is no evidence against them just like there is no evidence against god (possibly the best known leprechaun).
Heaven has been building a water-park. However, Heaven contracted out leprechaun laborers and the project has suffered severe logistical and budgetary roadblocks.