I did a search on Freereg to see baptisms for Mary

? bc 1796in Abergywngregyn . It revealed a few baptisms there and these were as follows:
17 September 1794 Mary Rowland, daughter of William Rowland and Mary.
19 October 1794, Mary Williams, daughter of William Williams and Catherine.
10 July 1795, Mary Evans, daughter of John Evans and Ellin.
11 March 1798, Mary Williams, daughter of Robert Williams and Margaret Hughes.
13 May 1798, Mary Roberts, daughter of Robert Roberts and Catherine Hughes.
I based the search on the age of Mary given on the census returns, she was 56 in 1851. The 1798 baptisms were probably too late to be for her.
It appears she was from Abergywengregyn, so I would have expected them to have married there.
Jo.