Who has bought the Heritage card and did you feel it was worth it? I'll be traveling to Dublin, Kilkenny, Cork, Ring of Kerry, Cliffs of Moher, Blarney Castle. I may want to see an occasional castle but not necessarily all the castles en route.
We bought the card on our first trip to Ireland and for us it was well worth the cost. To determine if it's going to be cost-effective for you you'll have to do the math (sorry). Add the individual entry fees for each thing you plan to see and see if buying the card is cheaper. One thing we liked about it, too, was having it already paid for before we left home.
You can check the cost of the sites you plan to visit. My wife and I went to Newgrange, Knowth, Trim Castle and Clonmacnoise within 24 hours of buying the OPW Heritage card and I think it paid for itself. Over the next 9 days I'm sure we turned a decent profit. Bottom line, you're only spending 21 euro each. If you are a tourist maniac, you might save 40 euros. One way or another it won't kill you if you don't buy the card. The Irish are probably more generous than the other tourist cards from what I've seen.
I will also say that when you show up at a site with the card and say something like "this was really a good deal" you get a smile from the people selling tickets. I think they appreciate people are coming to see the sites.