Gwenhwyfar means white ghost, a nickname for the owl.

Ireland and Gaelic Scotland are a different type of celtic culture to Brythonic Scotland and Wales. Later patriarchal gaels phased out the sacred marriage to the land. A ritual wedding to a sovereign/woman who was the living embodiment of the land, which included a stone in the ceremony, was replaced by vows made to the land directly,represented by a stone which made the vow binding and sacred.

The remnants in welsh of king making is the procession of the nine maidens that give the king the gift of a stone and his weapons. Eventually the land gave the king his weapon, through the sword in the stone. Once again, remove women from the equation.

There's a few ways the stone was used in the ritual. Some will have had new legends attached to them to make them "king stones" of later kings.

Petrosomatoglyphs are my favourite:-
http://en.wikipedia.org/wiki/Petrosomatoglyph#Footprints_in_England_and_Wales