Hiya Andy,
If you want a book that gives a good overview, you can't do better than Beckensall's 'British Prehistoric Rock Art'
http://www.tempus-publishing.com/bookdetails.php?isbn=0752425145
Ebay usually has a paperback copy floating about every couple of weeks.
As has been pointed out above, there's an appaling lack of books about irish open air RA. Whoever can be arsed to go photograph it all, write it up, then faff about with publishers, would be doing the world of cupspotters a big favour.
For a nice place to look for links twixt Irish and British RA, the Kilmartin area is worth a ponder. Inside the Neolithic Mind offers a well interesting take on the passage grave RA, but I don't think anyone out there in Professionalarchaeologistland is willing to stick their neck out regarding what the landscape RA is about. Too high a likelihood of being ridiculed I suspect. The best on offer is probably Bradley's Rock Art and the Prehistory of Atlantic Europe:
http://www.amazon.com/gp/product/0415165369/103-5792889-9557464?v=glance&n=283155