The ba captain had a JAR-FCL ATPL(A); such licences do not include embedded IMC privileges.
Incidentally, to exercise IMC privileges on an EASA aeroplane after 7 Apr 2014, as a minimum you will need an EASA licence which includes an IR(R). A UK ATPL(A) will not be acceptable for flying EASA aeroplanes after that date, except within the scope of LAPL(A) privileges - which are VFR only.
Ryanair had good reason to require their pilots to hold IAA licences as otherwise it would have been very difficult for LPCs to be conducted across a range of Member States due to examiner briefing requirements. Much easier to bring everyone under the one state of licence issue.
If you are quick, you can apply to the UK CAA for a change of state of licence issue and to convert your old UK ATPL. If it is still valid, you would then hold a Part-FCL professional pilot licence with an IR(R) - and this could also include an Aerobatic Rating if you can meet the conversion report requirements stated in CAP 804.