Send you P45 off to Revenue. Make sure you send it to the right place.
Include a statement from yourself saying that you are emigrating and that you will not be working in Ireland for the remainder of the year or indeed next year.
Ask for a refund of any tax overpaid in the current year and give them a forwarding address to send the cheque, if applicable.
Best to give parents/sibling address here as opposed to address in Spain.
How much you will get back is difficult to say, but needless to say you will have about 5 or six months of unused credits and SRCOP that should create a refund.
Other benefits - not that I can think of