For Irish property rental calculations, only certain expenses incurred during the rental are allowable against the rental income such as rates, rent, bank interest, repairs etc (and by concession accountants fees) which are not recovered from the tenant. If expenses are incurred outside of the rental, they CANNOT be offset. You may need to apportion bank interest, etc.
Items of a capital nature such as furniture - disallow and give capital allowances.
The same rules will apply to foreign property rental income in Ireland under Scedule D Case III or IV (depending on whether tax has already being paid in foreign country).
Regarding the calculation in Portugal, you will need an accountant probably to do this and ensure the correct returns are made.
Regards
Patrick