I think the original poster is looking for the marriage of Mary's parents.
I have already suggested the following -
"I checked all the Scottish and Irish marriages between 1860 and 1891 but, like you, I couldn't find them.
There is a marriage for Grace Anderson to Donald McNaughton at Tradeston, Glasgow in 1876. This sounds like a possibility, especially as they are at Tradeston in 1891.
If I were you, I'd check the 1876 marriage to look for the name Hood, perhaps his mother's maiden surname or her pervious married surname? Also check the address, you may be lucky and find that it is the same as the 1891 census, which would be too much of a coincidence to ignore".