It is a nice stone. Obviously widely used around Dublin.

This picture nicely shows a good example of a quartz seam:
http://www.themodernantiquarian.com/post/46370

This picture shows the way that the portal stone is 'veneered' with a layer of quartz:
http://www.themodernantiquarian.com/post/46371

Presumably the boulder was purposely split along a seam to do this. Several other monuments in the area use this feature.