I Understand The High retention rate is because you pay in 2 places
You do not pay in two places and I think you still didn't understand it.
There is no choice, it is covered by European laws and taxation agreements.
Your are based in Spain: you will have to pay employees social security in Spain. The company which pays you a salary will have to pay employers social security in Spain even if it is an Irish company. You are covered by the Spanish social security system.
You, a resident of Spain, receive a salary from an Irish company and will have to look at the double taxation agreement between Spain and Ireland to find out where you have to pay income tax. In your case its Ireland.