I'm only suggesting a possible line of research.
Mary & Jane may have been two separate persons.

The IGI does have a William & Jane Langley marriage in 1845 at Dublin, which would blow the theory.

Mary was born in Dublin and William in Limerick.
I suspect they married before coming to the Island.

Given the ages in the 1851 census, I think it unlikely that William Brown was grandson. However, if the age of William at burial in 1872 is correct, then his age in the 1851 census is too low by at least 6 or 7 years.

There is a baptism for a William Cowell Brown in 1845 with a mother Mary but I don't think this is the same person.