I find Ancestry most helpful for the hints of matches from others as so many use it.
Ancestry is useful for England, USA, Canada. Virtually all it's Irish collections are second hand from other sites that are free and we were familiar with using, before Ancestry later added them also as web collections. Census, civil records, all PRONI's collections, Belfast Burials.
Findmypast as it has newspapers is of more benefit than Ancestry.
Many users fail to appreciate the massive holes and gaps in church records for the 1800's and gleefully find and attach a R.C baptism for a Bernard Shaw despite him marrying in a Presbyterian church 50 or 80 miles away 22 years later. Evaluate and make your own decision before using.
DNA does not prove the accuracy of historical research it proves a link to someone living today even if they have selected the wrong ancestors.
Rathfriland is mostly in the Civil Parish of Drumgath
https://www.rosdavies.com/PHOTOSwords/DrumgathAll.htm partly in Drumballyroney
Rienroe is not a townland.
Moses McCrum died 1874 in Cullion, Clonallan Civil Parish
https://www.irishgenealogy.ie/files/civil/deaths_returns/deaths_1874/020655/7247755.pdf registered by his son David.
here
https://www.townlands.ie/down/iveagh-upper-upper-half/clonallan/clonallan-upper/cullion/A David was living there 1901
http://www.census.nationalarchives.ie/pages/1901/Down/Upper_Clonallan/Cullion/1250478/ and had a son Samuel (13). A John there had a son Samuel Mills McCrum also 13. Both Presbyterian.
The nearest Presbyterian churches would have been Hilltown (in Clonduff) or Rathfriland. No Presbyterian registers in the civil parishes date back to 1790, Clonduff (Hilltown) don't exist before 1845, and Rathfriland had 3 various start dates.
Refer to PRONI's guide to church registers for what records survive for his denomination, also consider Church of Ireland, over half lost for Ireland 1922.
Don't use the BMD indexes on Ancestry they have been superseeded now for many years by the register images and Newry Registration District is too big an area.