Griffiths Valuation for 1864 lists a Joseph Graham in Duvernagh townland. He had plot 37 which was a 12 acre farm. However your Joseph was a Tailor (according to the 1866 marriage certificate) so I am not sure it’s the right Joseph. I searched death records 1864 onwards (that’s when they start) for Newry registration area but could not find a Joseph who was a tailor nor a spouse Ann. Both might have died before 1864 of course or may have lived elsewhere.
In the 1901 census, all 17 Grahams in Divernagh/Duvernagh were Presbyterian. Was David also Presbyterian? (I know he married in the Church of Ireland but tradition was to marry in the bride’s church so that may just have been her denomination). The nearest Presbyterian church to Divernagh is Bessbrook. However unfortunately it’s records only start in 1854 so not much help.
I looked for possible siblings marriages around the same time but did not see any where the father was a tailor.