So if they lived in Bishops Horrible their assigned base would be STN (England & Wales), if they lived in Leiden then AMS (Netherlands), Paris then whatever ****e airfield RYR call Paris (France) etc., even if all of these were positioning to/from Germany to work from there their tax/SS would fall payable in England & Wales, Netherlands, France etc. ... Wouldn't it?
Yes, that's how I see it. It was an amendment to EU legislation on coordination of social security that came into effect in June 2012 - for flight crew the assigned base is used as determination where SS is due. People based pre June -12 may pay Irish SS for a transition period of up to 10 years, unless a change is requested by either employer or employee.
Ironically you mention France, one country that has no Ryanair bases. Guess why? They did not want to play this silly game of Ryanair with based crews on Irish contracts following Irish labour laws.
I don't know the details of the court case to be honest, but before I left I heard people based in Germany moaning words to the effect of 'if you're gonna be fully tax compliant in Germany you'll be paying 60% + in taxes'. That's nothing they tell you in East Midlands at the induction week, but something you figure out as you go along.