Preps here are attached to grammar secondary schools. They aren’t for everyone, and not always good value for money as some are in the same locations as excellent state primaries. But definitely worth having a look at their websites etc.
PP have mentioned South and East Belfast so there’s-
Victoria Prep (south Belfast, girls only, attached to Victoria College)
Inchmarlo (south Belfast, attached to RBAI, boys only)
Fullerton (South Belfast, attached to Methodist College, mixed)
Downey House (East Belfast, attached to Methody as well, mixed)
Cabin Hill (East, attached to Campbell College, boys only)
Penryn (East, attached to Strathearn, girls)
In Lisburn there’s Friends School Prep and Wallace Prep- we’re looking at Wallace for DD but tbh our village school is fantastic so it might not pay off for us!
They’re all much of a muchness fee wise- none about 4.5k per year last time I looked.