It's great that you are planning to visit both Dublin and Belfast in March! ☘️
As long as you enter and leave the island of Ireland through an airport or ferry port in the Republic of Ireland, you don't need a passport to visit Belfast and Northern Ireland. This page
has more information on passports and ID documents.
What have you got planned in Dublin
so far? I'd love to provide you with tips if needed.
Please note that I can also help you in French if you prefer. 😊