RYR-738-Jockey, you are correct, an FCI was issued about 12 months ago stating the need for confirmation, but come on, give me a break even if Ryanair denied having received your resignation letter they cannot in any way, shape or form stop you from leaving the company. If you also read the three points I've mentioned you'll note that I state you need to keep a copy of your letter, along with a signature and/or proof of posting so there cannot be a 'legal' denial from Ryaniar.
As you probably know, we are losing a number of pilots from FR & have been doing certainly for the past couple of years. It really doesn't take an Einstein to work out how to terminate his/her employment, or a question on here!