I work in Norway and live in the UK.
As Norway's not, quite EU. They have a tax agreement with the UK.
I complete certain forms and pay social security only in one of the countries. Regarding tax, I actually pay tax in both the UK and Norway. Then use tax equalisation to claim tax back.
I am surprised that if someone lives in the UK and works for Ryanair, they only need to pay Irish taxes. Especially those based in the UK.
Obviously there must be some agreements between UK and Ireland or perhaps EU rules that allow you to pay tax in other EU states, when living in another. Although I am not versus in this and would suspect EU countries you live in, may challenge that in a European court or would be already to stop this.