Certainly agree with the bit about OS maps and if you're not sure how to use them (I'm getting a bit better) make sure you go with someone who can. We relied heavily on OS maps when in South West Ireland recently and spent one afternoon trying to find three stone circles shown on the OS map as being on the hills around the town of Bantry. What maps don't show is 'private property' and on that occasion we got caught in the act of climbing a locked gate - the land owner was quite cross and said she was forever turning people away who came looking for a non-existent stone circle. We never did find out if it existed or not.
If going to an area (such as Ireland) you are unfamiliar with it is a good idea to check out local bookshops before you start. I discovered some publications by someone named Jack Roberts who had an amazingly comprehensive local knowledge and had done drawings of all the sites he visited. Most of these publications were compiled/written in the 1980s and I have been unable to find out much about the author who does not seem to have an internet presence at all. Local bookshops! Don't forget local bookshops.
Good point about local bookshops agreed, don't you (a song title I've heard) forget about the locals as well :-)