The Hill of Slane goes back – prehistory, probably as far as the Neolithic. Drone Girl Ireland provides us a great video tour of the hill and the castle
In ancient Dindshenchas mythology, Fir Bolg king Sláine apparently has been buried on the Hill of Slane. In Christian history the hill was established after the St. Patrick lit the first paschal fire. The hill remained long after St. Patrick appointed Erc as the first Bishop – a monastery survived on the Hill of Slane even after successive raids by the Vikings. Still is a mecca for pilgrims to this day. Read more on Wikipedia.