When we were planning our trip to Ireland the majority of places we wanted to see would link us to a website for the Irish Heritage Card. We looked into it further and found that it would work perfectly for us. It's like a pass you buy it either thru the website (we had to mail an order form in) or you can purchase it at the first Heritage site you visit. It cost 21 Euro which I think ended up being around 35 or 36 US, but 95 % of all the historic sites and gardens we were going to were Heritage sights so we got in for free. It paid for itself in the first 2 days. It is not valid in Northern Ireland but there are places all over the Republic of Ireland. So if you're planning a trip to Ireland and you have an idea what you would like to visit check out the website at http://www.heritageireland.ie/en/. If you're going to some Heritage sites it may make sense to get the Heritage card, also it is valid for 1 year.
On a side note if you're going to Scotland they also have a card issued thru the National Trust of Scotland, a little more pricey at 37 pounds but again if you're planning on going to several NTS sites well worth it, you also get a parking pass for sites as well as 3issues of Scotland for You, a magazine about the Trust sites and a book with all the Trust listings, directions to the properties and opening times. Their website is http://www.nts.org.uk/web/site/home/visit/Visit_home.asp?NavId=2004&NavPage=26