Lauretta,
If Samuel Brown was a farmer then he should be in the tithe applotment records. There’s only 1 in Magheralin parish and that’s the one I have identified in Ballyleny. The one in Tullyloob that Patricia has helpfully identified was very close by, and should certainly not be discounted, but was in the parish of Moira. If yours definitely came from Magheralin then he’s more likely to be the one from Ballyleny, but an open mind is required in case of mistakes.
Nearest Presbyterian church to Magheralin is Moira. Unfortunately it has no records before 1866. So if the family was that denomination, and attended that church, tracing them will be harder. That church does have a graveyard (in Carnalbanagh East) but you should check the other cemeteries too as folk weren’t always buried in the graveyard attached to the church they attended. Presbyterians generally didn’t keep any burial records so it may just be a matter of going round the graveyards and searching. The Church of Ireland did keep burial records but only for their own congregation so other denominations buried in their graveyards (which were open to all denominations) aren’t recorded.
There were several Samuel Browns in the Magheralin/Moira area, so you’ll need to take care to identify the right family. There were still 2 Browns farms in Tullyloob in 1901:
http://www.census.nationalarchives.ie/pages/1901/Down/Moira/Tullyloob/1243220/
http://www.census.nationalarchives.ie/pages/1901/Down/Moira/Tullyloob/1243219/
You may notice both households were Church of Ireland, so if your family were Presbyterian that is hint that they are not related to your family. (As well as living in Moira parish rather than Magheralin).
Map here which shows the townlands in that general area. Townlands to the left of the green line (which includes Ballyleny) are in Magheralin parish. Those to the right (which includes Tullyloob) are in Moira parish.
http://freepages.rootsweb.com/~rosdavies/genealogy/MAPS/MoiraMagheralin…
I had a look for Glenny households in that area but didn’t find any in the tithes. Brown and Martin are common names though. 2 Samuel Browns in Magheralin on Rosdavies website, both from Ballyleany/Ballyleany:
http://freepages.rootsweb.com/~rosdavies/genealogy/SURNAMES/B/BrownSW.h…