Christopher Smith appears in the Valuation Revision records for Copney till 1882, suggesting he died around that time. He was replaced by William Smith and then in 1887 Alexander was also listed as joint tenant. The only Christopher Smith/Smyth death around that time in the Omagh registration area was on 11.4.1875, aged 67, so presumably that's the correct family. You might want to look at that in case it names his wife. It should also tell you whether she died before him or after him, which may aid your search.
You can view the original certificate 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.
You haven’t said what denomination the family was, though I suspect Presbyterian. Searching church records for Christopher’s marriage and his children’s baptisms would be the normal way forward. However in this case the nearest Presbyterian church appears to be Sixmilecross and unfortunately it’s pre 1900 records were lost in a fire. Do you know where the family is buried? Perhaps there’s a gravestone with Christopher’s wife’s name?