My husband and I went to Ireland in the spring of 2004 (wonderful trip), and we purchased the Heritage cards at our first stop in Glendalough. Most of the sites we visited on our trip accepted the card. For us, it worked very well.
I'm not sure how to do the hyper link (need to have my kids show me), however, the web address is listed below.
Marcia, your link worked perfectly. If you click on the link and go to the site map, they let you organize things by region...which can help you match Heritage Card sites with your itinerary.
It doesn't hurt to flash your Heritage Cards at non-Heritage Card sites. Most places just tell you they aren't part of it, but the fort at Duncannon gave us a discount just for having the card.
In addition to the great advice from Marcia and Corey (good tip about flashing the card everywhere), this is the page that lists all the attractions that the Heritage Card will admit you to: http://www.heritageireland.ie/en/A-Z/