I think it's just a natural bit of erosion in the stone there. That cross appeared in one of Julian's videos by the way!

Have a look at these http://www.megalithomania.com/show/site/110

The area to the south of Dublin is home to a peculiar set of grave slabs that have cup marks, herring bone patterns, lozenges and CnRs in their designs. Quite lovely.

Here's a couple of the others

http://www.megalithomania.com/show/site/873
http://www.megalithomania.com/show/site/629