I found out a little more about Mary Tainsh by cross-referencing her maiden and married name in the Scots Origins death index. Mary died at age 75 in 1915 in Alva, Clackmannan, Scotland. Therefore, this would place her birth around 1840. I will order the certificate sometime in the near future to see if it contains any additional information (hopefully at least her place of birth).
If Mary and george were born 15 years apart it is highly likely that they had other siblings.