I looked the John & Rebecca’s deaths up on the GRONI website and both appear to have been living in Co. Londonderry when they died. John’s date of death was 23rd May 1864 and Rebecca’s 19th March 1865. You can view the original certificates on-line on the GRONI website, using the “search registrations” option:
You will need to open an account and buy some credits. It costs £2.50 (sterling) to a view a certificate.
The addresses and informants on the death certificates may help you start tracing the family. (Put that information on this board and we’ll see what we can do).
And if they were buried in Londonderry, which cemetery? Have you checked with the cemetery authorities to see if they have any records of next of kin, and who else is buried there?
I looked for a John Allen who died in Londonderry in 1880 but could not find one. I also searched for Derrylough in Co Donegal, but again without success.
I did find a John Allen of about the right age who died 4th Aug 1882 at 7 Ann St, Lecky Rd, Derry aged 63 (so born c 1819). He was a widower. Informant was his grandson George.