Very true, and I would probably have made a very similar mistake myself. Incidentally, I don't believe I was even asked for ID on my last two UK domestic trips.

Call your credit card company and let them hash it out with Ryanair, especially if it is not completely clear on your booking confirmation, or on the website, that you require ID.

Ridiculous, IMHO - but sadly I've come to learn that booking with these people is always a risk, and wouldn't consider it for crucial trips anymore.