Does Ireland have a general pass for all national parks, castles etc, similar to Canada's Discovery Pass or the US National Park Pass. If yes, where can I get it?
The OPW Heritage Card is probably what you're thinking of. It's well worth the money if you're going to be in the country for more than a week or two. Can purchase one on the website or just buy one at your first stop.
I agree with Robert that the Heritage card is well worthwhile. As a senior our cost was about $30 and we figured we got over $100 in admissions for it. I might suggest buying it prior to your trip online so that you can book your tours/admissions ahead of time since many places like Kilmainham Gaol, Bru na Boine fill up.
The Heritage card is very good value. We found we went in to many attractions we wouldn't have bothered with if we'd had to pay for them! (It is also useful as many sites have toilets....)
In response to JimK about buying in advance.... you don't need to buy in advance and you can still get tickets to the major sites in advance. When you go online to purchase a ticket for a place like Newgrange, there will be an option for 'OPW passholder' (or something like that). Just take that option and you won't be charged, but when you get to the site, you have to show your ticket AND your OPW pass. We had an OPW pass (seniors) and found it did pay for itself.