Hey David,
How nice that you are coming over this summer!
Not to worry, your Belgian ID card is sufficient to visit Belfast and Northern Ireland as long as you arrive and leave the island through the Republic of Ireland (which you'll do by arriving and leaving from Rosslare Harbour). We have a few more useful details on travel documents
right here.
Have you figured out your road-trip itinerary just yet and is it your first time in Ireland?
Groet,
Antoine