You should only do the instructor rating if you want to instruct.
Don't complete a JOC/FI comparison on the basis of money as it defeats the point of instructing which is to impart your passion to someone.
Also if you want to instruct at this school in Ireland you should definitely complete your CPL/IR there, not go to Jerez then come back for the instructor rating.
If you complete it all one stop your more likely to get a better product, service and become a better pilot, and instructor if you are tuned to an harmonised approach.