Via Atlas Obscura:

Some say that Puck’s Castle, in Rathmichael Co. Dublin, was built from sacred stones culled from the nearby Bearna Dhearg (or “ringfort”), but little is really known for certain about the structure today. Because of the “sacred stones” story, Puck’s – an English derivative of the Gaelic “púca” or “pooka” meaning ghost or spirit – is said to be haunted.