Peter, This is of course something we had to check out for other sites...
Kilkenny for example has so many historic buildings, and is the first stop outside of Dublin so we contacted Fáilte Ireland, who confirmed that there is no restriction on photographing building exteriors. However some OPW sites (Kilkenny castle for example) don't permit photography inside.
Not all buildings in Ireland are under OPW control, but they do have about over 750 sites (see From the perspective of our tours we don't ask if the customers will be selling the images, the majority of people come from camera clubs and are doing so for their own use.
Technically Clonmacnoise could be in a tour of ireland for the purpose of taking photos, we can't control what's done with the photos, as far we're concerned they're for personal use.
To be honest, it's something that would be nigh on impossible to police. How many shots are taken in trinity every day/year, how do they check if they are sold?