The hubby and I did a self supported tour in Spain a few years ago. I did most of the planning over the internet, with the help of a Michelin map of Spain. We booked hotels in the larger towns and found a place to stay when we got there for a few of the really small ones.
For a trip planned quickly, with no knowledge of the Spanish language we did very well. No big safus that I can recall, and I would guess that Ireland would be easier - people speaking English there and all.
We did a hostel/hotel based tour and carried minimum gear - two sets of street clothes, two sets of cycling clothes, tights and jacket, one pair of street shoes, toiletries, camera, snacks, book.
We did take our own bikes, as we had heard that renting a bike in Spain was next to impossible. Our biggest challenge was getting the first hotel manager to understand (in our next to no Spanish and his next to no English) that we wanted to leave these big boxes (we rented travel cases) in his hotel's storage room for two weeks, but that we would be back. He didn't understand at all until we got the bikes out, assembled them and packed all of our stuff on them, what the crazy Americans were up to.
Otherwise everything went quite smoothly - the roads we chose were all great, we had no problems finding rooms in the towns we hadn't prebooked and every one was very very friendly. I would very definitely do it again.

On the other side of the coin - we did a walking tour of the Yorkshire Dales some years ago and chose to go with a semi supported tour - they gave us maps and transported our bags (no guide or group) and that was a great experience as well. We didn't have to worry about anything besides walking to our next destination.