I think Paul is actually referring to the distinction between the Flight ID/radio callsign on one hand (which for Ryanair, and many other European airlines, is often an alphanumeric ID) and the flight number shown on tickets, airport departure boards, etc, which is usually just a number and may bear no relation at all to the callsign.
For example RYR42AQ (Bristol to Dublin) = FR509
Paul, if you do a forum search on "alphanumeric" you will see that this topic has been widely debated, but with no conclusive solution so far.
We're told that the network is running a process that ties the alphanumeric Flight ID to an arrival and/or departure airport (from which it is then fairly easy to work out the ticketed flight number), but I've yet to see any evidence that this process is producing results, or even a full explanation of how it is meant to work.