Zander,
If you find this Glenvarlock Street Robert Brennan is yours, let me know as I have a lot of information on the Brennans of this part of the Castlereagh Road/Beersbridge Road. Their ancestors were Co Antrim but not as far north as Dervock- Whitehead, Ballycarry, Islandmagee so this may be a coincidence.
Good luck,
Platespinner