Just that, really... I have a Passport Card along with my normal passport (what the DFA calls a "Passport Book") and it's great. It's basically like being any other EU citizen travelling around on just their ID card. It fits in your phone cover and if you lose it, well, you still have your actual passport.
It costs €35, but if you order it when you renew your passport it's only €25 extra.
I was wondering how many people have taken it up, not least because not many people I speak to seem to know about it and I'm hoping they don't abolish it for want of popularity!