The Ancients of what is now County Carlow seem to have had a 'thing' for grooves as well as many grooved stones <i>a la</i> the Devils Arrows

http://www.themodernantiquarian.com/post/29909
http://www.themodernantiquarian.com/post/26731
http://www.themodernantiquarian.com/post/29915

they also had a thing for rutted capstones

http://www.themodernantiquarian.com/post/29907
http://www.themodernantiquarian.com/post/4285

Both these tombs are riverside and I think that the capstones were taken from the river beds.