If Francis & Mary were born and married in the RC parish of Longfield or Langfield, then a problem you face is that that parish has no baptism or marriage records earlier than 1846. This could be why you were unable to find any record of them when you contacted the parish priest.
I looked for McNab households in Mullonatoomog c 1885, and Mullagharn c 1890 in the revaluation records but none is listed. That doesn’t mean they weren’t living there but just that either they were lodging with someone else, or they didn’t live there long enough to be picked up by the Griffiths clerks or that they had a cabin of too low a value to be listed.
Labourers often moved around a lot and can be notoriously difficult to trace. They are unlikely to have any gravestone either, unless someone else paid for it.
It’s sad that there are no Poor Law admission records for Omagh but in general Irish Poor Law records don’t contain as much info as the Scottish ones (where you get a whole file) and so even if they did exist, you might not have got any more information about the family than you already have. Irish ones are mostly just a line of information in the admissions register, giving personal details plus the reason for admission, and date of departure/death.
A little general information about Omagh workhouse. The infirmary is the only bit still in existence today.
http://www.workhouses.org.uk/Omagh/You don’t really get a lot of information on an Irish or English death certificate, compared with Scotland and some other jurisdictions. Can be very frustrating.
Do these dates for the 2 deaths you have found tie in with any Scottish marriage or death certs at all? For example, if one of the children married or died in say 1887, the cert ought to give the mother as dead and the father as alive.