Surely then, if your Spanish based, you pay your taxes and social security solely in Spain not Ireland? Would there not be some bi/multilateral EU tax agreement
Don't be so sure, it seems Ireland is getting most of the dough anyway. Those pesky Irish accountants tax it at source before they pay it out to you. Then it's up to you to declare that in whatever country you're tax resident. Based on the bi-lateral tax agreement you may come out paying no more taxes. Still, the money went to the limericks and not to your kid's school education.
Note that I have no first hand experience with Spain, but this is how it works for many Brookfield/Storm contractors in other European countries. Probably same for Ryanair employees as those are Irish contracts too. I think only Italy has a variation to that where Italian tax is deducted for domestic flights. This was a few years ago and might have changed.