I remember seeing this when Rockandy posted it first and sayin to meself 'bullaun'. Looking at it again, the right-hand basin is a bit like this http://www.themodernantiquarian.com/post/53129/images/cairn_x2.html a hole carved by army sappers to erect a flagpole in a fallen cairn orthostat at Loughcrew. The sides of the Greenlee Lough basin look a bit sheer. Most bullaun basins have a gradual, smooth widening towards the top of the basin, as if they were carved in a mortar and pestle fashion rather that a hammering or chiselling.