Before our first trip to Ireland several years ago, I did a Search on this Forum for the same question. I was about to buy the Ordnance map, but I saw a comment from another poster who recommended the AA Road Atlas of Ireland as being easier to read because of the colors used. I followed that advice and was pleased with it. We drove all around the island in 2 weeks and I was only "lost" once despite the many rural roads we were on. Or maybe not - the "lost" adventure was a very narrow farm road, but then so many of the roads there are just that. We did get to where we were going. It was the 5th Edition Atlas and claimed to be based on the Ordnance map. It contained a mileage chart, a map of Counties, small maps of some of the bigger cities, a "motorway" map ( like a US Interstate map that is compressed to show exits & interchanges between various points), an index of towns and villages, and even a guide to freshwater fishing. I think it was about $12-$13 at Amazon. We were happy with it. It's been back to Ireland twice more with no complaints. Have a grand trip, Ireland is splendid.
If you Search on this topic you will find the recommendations of other posters.