There are two things you'll need to know about the bus credit-slips in Dublin. The first is that you only get one if you tell the driver the fare you're supposed to be paying. As a newcomer into town, this is a bit troublesome, since you're not sure what any of the fares are. Anyway, the driver figured it out, and I got the credit. There are offices of the bus company in downtown, and it turns out that if you stop by any of them, they'll happily give you cash for your credits. The trick is, that you have to have saved them. I'm not sure if it's worth the effort, but you can cash 'em in. We bought Rambler tickets at a convenience store, so I only had one credit-slip to worry about.

More information is available in the Irish Times article Passengers urged to get their own back.