We just returned from 3 weeks in Ireland. Purchased Heritage Card on Day 1 in Dublin. We haven't done a cost/benefit analysis, but probably would have spent a bit less on an "ala carte" basis. Still, it was convenient to just pull it out and walk into the sites where it was accepted. If you know your likely stops, it's easy enough to make the calculation from info on the website.
Also, on the basis of this Forum, we'd made reservations (and paid for admission) in advance to visit the Kilmainham Gaol in Dublin. Upon presenting the Heritage Card, the nice ladies at the Gaol reception desk refunded our pre-paid admission in cash on the spot -- an unexpected courtesy -- but an accurate preview of what we would encounter on a daily basis going forward. You'll have a great time and, btw, Kilmainham Gaol is a grim but moving and educational site to visit.
(Oops, I was writing as Robert was posting; sorry for the duplication.)