Hi Liz,
According to 1851 census your Thomas' are living in the centre of Morriston the nearest burial grounds were Horeb (now demolished) & Seion Newydd, both non conformist chapels.
Unfortunately the burial records for Seion Newydd are nowhere to be found

I can't find any burial records for Horeb listed at Swansea archives
The nearest C of E Churchyards were Llangyfelach & Llansamlet.
I can't find a burial for a Jemima Thomas at Llansamlet
There is a Jemima Thomas, of Morriston, buried at Llangyfelach on 20 Nov 1859 aged 66 years.
I've checked my copy of the MI's of Llangyfelach - plenty of Thomas Thomas' but no mention of a Jemima.
Non