I'm confused; There is Ballyrainey in Dundonald Parish and Ballygrainey in Hollywood Parish in Castlereagh Lower Barony. Is this a spelling mistake or are they two different townlands?
These are two different townlands in two separate but adjacent civil parishes. There's another townland named Ballygrainey nearby in Bangor civil parish.
There are many similar and duplicate placenames around Ireland, not just townlands but also towns, villages and parishes. There are also some townlands that cross civil parish or barony boundaries.