Hi deb
I think the best bet we have for confirming that Mary was the daughter of james Bryson ( other than her marriage which names James as her father and the evidence that her mother was Ann McConnell) is if any Bryson descsendants have a match to any of the Bell family lines. Though any of her brother Robert's descendants could be a match to bell descaedants through Ann McConnell only.
I only looked for children of Mary Bryson and William Samuel Bell born between 1864 and 1870 (there was another daughter Mary in that period) but I did see a tree in Ancestry, owner had Watkins surname, which listed about 6 children ( though was missing Ann b 1865). Oldest son was William (apparently born c 1861) and he was the main beneficiary in Mary's Will written 1896, which named 2 farms belonging to her deceased husband William Bell, one at Ballyclaverty and the other at Ballywoodick. My relative, Andrew Gawn of Halftown was one executor and her son William was the other.
Mary's birth year could easily be an estimate based on her age at death or possibly the 1901 census, I do not know but neither is necessarily accurate.
When Mary Bryson of Ballyclaverty, daughter of James married William Samuel Bell in May 1858, she was of full age so her latest possible date of brith should be about May 1837 but there is no reason to belive it could not have been earlier.