Right. Now I've actually looked at the picture <ahem> ...

It's a mid form of Chi-Rho cross. The bit at the top is not a complete circle, but just hooked around. The cross as we know it started out as a combination of the Greek letters Chi & Rho - the first two letters of Christ's name in Greek (Christos being a Greek name). These crosses looked like this: http://www.ladydorothea125.net/crosses/Cross9.html

As time went by the X part narrowed to become a horizontal bar and the curly bit at the top got smaller like this: http://www.ancientstoneart.co.uk/gallery/scot/s8echoid.htm

When looking at Irish Cross Slabs the style of the cross is a good way of dating it. For a lot more on the development of crosses I can recommend "Symbolism of the Celtic Cross" by Derek Bryce (don't know how available this is these days).