Any other ICAO CPL/ATPL licence (for instance a FAA, Canadian, South African etc etc ). Believe it or not there are some commercial licences out there that do not meet ICAO standards.
Basically they are wanting people who the IAA can issue validations to as you may not have an EASA licence. So perhaps ask the IAA if your licence can be validated. The process then would be for you to obtain the EASA licence while already employed and working for Ryanair using your validation. As I believe these are only issued for a years duration and while they can be renewed they are in place for this reason and not to be used/renewed indefinitely in order to avoid obtaining the EASA licence.