The triple spiral or triskele is a Celtic and pre-Celtic symbol found on a number of Irish Megalithic and Neolithic sites, most notably inside the Newgrange passage tomb, on the entrance stone, and on some of the curbstones surrounding the mound.
 
What the symbol meant to the pagans who built Newgrange and other monuments is unknown. As Christianity came into the forefront in Ireland before the 5th century, AD, the triskele took on new meaning, as a symbol of the Trinity: Father, Son, and Holy Spirit, and therefore, also a symbol of eternity.
 
It's popularity continues today as a decorative symbol of faith for Christians of Celtic descent around the world.