Hard to know isn't it

I would say from what you have that it is the same couple. Why the use of Mary Ann twice, I can't guess.
Mary Ann who married Henry Henderson showed as 18 when she married Henry in 1871, although her census entry showed as 16. Her birth place in on the 1891 census gave Co Cavan as her birth location. Baby Mary Ann above showed Co Fermanagh, Omah as her birth place.
Regarding Mary Ann's occupation, on the 1871 census it showed as Factory Guider. On her marriage entry it showed as 'Worker in a Cotton Mill'.
What is the address on Mary Ann's marriage reg? I can only make out Square!
Monica