You can camp just about anywhere you please. I don't know about the legal niceties but that hasn't stopped me so far.
Any place along the western seaboard is great. Practise discretion (stealth) if you are near dwellings and such but the seeking of permission to camp on private property is often fruitful. On high ground generally a lot of the land is commonage so there is nobody really to ask, just don't camp near cows. Sheep-good, cows-bad.
Google Earth and Bing Maps aerial photos and maps are rubbish for most of the country so a database is not of much use to you.
The larger national parks are not patrolled often so camping in them off the beaten track is not a problem but please do not light any fires.
Asking the locals about camping spots is a good way to find a place to pitch. My personal favourites are my local peninsulas and islands in the far Southwest of the country,
http://en.wikipedia.org/wiki/Category:Peninsulas_of_County_Kerry
Irish OS maps are available from
http://www.stanfords.co.uk/. If you need any questions answered about a specific place, please feel free to ask me.
Happy trails...torc