Took a ferry from Doolin as well. Keep your ticket, you pay one price for the round trip.
Actually, I got lucky. Turns out I ran out of money completely on the Aran islands and it's pretty much cash only out there, as far as I remember (or I should at least say credit card machines aren't near as prevalent as here). I think I had lost my ferry ticket somewhere but the guy running the ferry remembered me so he let me on. I was so afraid I'd be stuck there (which would actually have been pretty cool, they're beautiful islands).