Lorena González García wrote:
I have been hired by a company with fiscal headquarters in Spain (my country of residence)...
Spain has special rules that are different from other EU countries, so I recommend you consult a bookkeeper.
...but it is registered in the ROI, so I don't know if I should apply VAT on my invoices.
Find out what address they want you to put on your invoice.
A general rule is that the country that you put on your invoice is the country that determines the tax regime. If I were you, if the address is in Spain, then I'll apply Spanish VAT. If, however, the address is in Ireland, then I would write "VAT shifted" and submit an intra-Community return when tax season comes.
But how that works, would depend on Spanish tax law.
I am thinking of registering in the ROI, so both being in the ROI I should not apply it, right?
For tax avoidance (i.e. running as a company in another country where you are not a permanent resident), you really need to consult an accountant that specializes in international tax law.