Personal tax liability is only a small part of it.
What Ryanair & their business model have ( & need to continue to do for the whole thing to add up) done is avoid paying social charges for their employees in the country where they work ( & effectively reside/ are employed in , never mind EI-??? on the side)
This has been achieved by an option that allows aviation workers to be classified as working where the "core business" of their employer is situated (Dublin in this case) & is
AFAIK legal in Europe.
The French, & now Italians, have challenged this using their national law, hence why Ryanair closed MRS, & now. . . . . Italian bases ? ? ? well, the cost to FR would be enormous, what the Italian authorities need to do is weigh it up against the income they receive by FR feeding in millions of pax each year.
Saving these social charges is a fairly fundamental part of their business model, & a "half-reality" most of us have been saying for years, would have to be resolved one day.
Country by country the chickens are coming home to roost, as most European countries teeter on bankruptcy & scratch around trying to get some income wherever they can identify a possibility.
You can run, but you can't hide for ever.
Edited to say, for the avoidance of doubt they "have" been paying social charges (doubtless at a much more favourable rate than available in France/Italy etc) but in Ireland, for full time employees.
However. . . as 75-78% of pilots are now "self employed service providers"
you can imagine they have cost FR nothing/nowhere as regards social contributions. For sure they have done a cosy deal with the Irish authorities involving something along the lines of "you turn a blind eye to the fact that we pay no social charges here for these lads, & we in return will tell them they have to set up a limited company & pay their taxes in Ireland, regardless of where they live/are based"
Unfortunately for FR, other Govts are not "in" on this deal, so the arrangement may not do them much good further afield.