Hi C.!
You are correct! As long as you arrive and leave through the Republic of Ireland, you don't need a passport even if you decide to explore Northern Ireland for a day or two (or three). We got
a page with more information about travel documents that you may find useful.
Are you planning to explore Northern Ireland as the main part of your trip or just as a day tour? Let us know if you need any suggestions in this part of the island. 😊
Cheers,
Antoine