http://www.megalithomania.com/show/image/1059
Maybe the earliest sundial there is? There's a similar stone on Patrickstown Hill at Loughcrew at Cairn X (I think). However, this stone doesn't face south and the one at Loughcrew is inside a tomb.
The stone in this photo http://www.themodernantiquarian.com/post/43664 is also thought to be a sundial. There is a cupmark in the upper surface of the stone and a large curved groove. I really should have a better photo of it somewhere.