The Church of Ireland parish records for Buncrana (Fahan Lower) start in 1817 so a trawl of those on rootsireland or in the copy held in PRONI in Belfast may reveal John’s siblings.
I noted this death notice:
December 19 1856
At Buncrana, on the 7th inst., aged 60 years, after a lingering illness, ELEANOR, the beloved wife of MR ABRAHAM DINSMORE
I don’t see a death for Abraham in Inishowen, post 1864 suggesting perhaps that they had died or moved away before that year. (You might find them in the Church of Ireland burial records). Looking at deaths for Inishowen as a whole, the only one I noted was a George Hastings Dinsmore who died there in 1897 aged 73. There weren’t any others. Suggesting younger generations have moved away.
You can follow the Griffiths records forward to see when Abraham vacated the property in Townsend St, using the revaluation records, which are kept in the Valuation Office in Dublin.