The maps I use, from which the OS co-ordinates are taken are the basic Ordinance Survey Ireland maps which are available online and in most bookshops. They are cheap enough and cost about €8 each. The cost mounts up, however, when you buy multiple maps. The area I cover which, at present, is just 6 counties and requires about 20 maps. However, there are no better maps as they include everything. Every track, road and building is marked as well as the majority of antiquities so these are essential for finding sites. Road maps are fine to use if you have a handheld GPS and have co-ordinates but with an OS map it is hard to get lost. Reading them is also easy and each map comes with instructions on how to work with the co-ordinates system (which is pretty simple). The 3rd edition of the OS maps are now being published and are the most current. The 2nd editions are still good though.
I have created a map on Google Maps which contains all the sites I have visited and can be accessed by clicking here.