Thanks Haydon for looking.
I only know what you have already seen from the census papers.
The family were living in the Porth area at the last known address in Hafod 1881 (Llanwono parish).
Maria is a fairly unusual name so that's the one I think might be easiest.
I don't know a lot about the branches of religion, but they could have been members of the local Chapel. (Baptist?)
I also don't know the area at all, living in Sussex! If you are local can you suggest any other burial sites.
The other option of course, is they are buried in the Llantrisant area, as that's where they were when first married.
Cheers.