The easiest way for you to find out the legal requirements for marriage in Portugal is to contact the Portugese embassy in Dublin.
Embassy of Portugal
Knocksinna Mews
7 Willow Park / Westminster Park
Foxrock
Dublin 18
Tel: 01 289 4416/01 289 3375
Fax: 01 289 2849
If your marriage is a legal marriage in Portugal then it will be recognised here in Ireland on your return. You will have no need to register this marriage on your return to Ireland.
Sometimes, people get married abroad in religious ceremonies only - that have no legal status either abroad or here. The embassy will be able to give you information on the legal requirements you need to fulfil - but I might also suggest you have a read through this document on OASIS here:
Congratulations to you both and best of luck with your wedding arrangements.