"How much recorded open air rock-art is there in ireland? From my quick review there doesnt seem to be that much, so would there be enough to put together a book"

If the British mainland is anything to go by I would lay a pound to a pinch of shite that there's a lot more to be found in Ireland than has currently been recorded.
Mr Beckensall began by publishing small booklets, nowt too grand just lots of descriptions of sites, drawings, photos and maps.
I guess rock art is one of the few areas of research where an amateur, with a good eye and a blue bucket, can make a meaningful contribution not only to the discovery of new carvings but to the interpretation of their place in the landscape.

If you’ve got legs, time & a bucket that’s all you need. In the UK you very rarely find an isolated carved stone, where theres one, theres usually others within a few hundred meters.

For starters, I’d start off by checking the surrounding area where known carvings are. But beware, if you find a new one, you’ll be hooked…… RA is like crack cocaine:-)