One thing to be aware of, availability of automatic transmission cars is usually fairly limited in Ireland except at the major airports, and they always cost quite a bit extra. If you can't (or prefer not to) drive a stick, factor that into your plans/budget. There are small car rental companies scattered throughout the country that are quite reliable and often more affordable than the major international companies if you only want to rent part of the time, but most of them have very few automatics available. Also, gasoline (petrol) is immensely more expensive than in the US, so unless you have money to burn, don't accept an upgrade to a larger vehicle (and they are also much trickier to drive in rural areas than a smaller vehicle. Stacked stone walls with no shoulder are not forgiving in that respect.)
Also, if you're not frequent int'l travellers, be aware that different countries and even airports within countries sometimes have vastly different tax rates when it comes to international air travel. When planning your trip, be sure to price out the airfare with some different airports, and be aware that sometimes you can save quite a bit by arriving/leaving at one airport over another. We've always found it much cheaper to both arrive and depart from Ireland, and take internal flights to/from the UK; it would seem counterintuitive to add an extra flight, but the tax difference can be substantial for a family, especially if your UK airport is LHR. (Also be aware that the luggage limits for intra-European flights are generally more restrictive than in the US.)