Mine is 8 years old and it's fine & I've used it a lot for the US in the last few years. Longer than 6 months I think is required. There's a good thread here from last week....do a search it'll be around.
Longer that six months isn't required, there is a waiver for Irish citizens. As long as its valid for your return journey it's fine. I checked this out last year, I went in Nov and my passport expired in March this year.