Thanks, Elwyn, you're so helpful to everyone on message boards! Yes, I'm checking Church of Ireland registers and cemeteries as well as Presbyterian, since we know that Presbyt. families were dissenters from the established church but often recorded life events in that "official" church.
Some of the Armstrongs in my ancestor Hanna Armstrong's family were Church of Ireland all along, and the Donaghmore parish registers (St. Michael's at Castlecaulfield) are full of Armstrongs. I don't know about the Browns. Too many of them to tell yet!