Danny,
There are 2 Deerparks in Drumragh – Deerpark (Clarke) & Deerpark (McCormick). Griffiths Valuation for those 2 were compiled in 1860. So if your family had all moved to Scotland c 1856 it’s not very likely that they’d be in Griffiths in 1860.
The Mellon household in Deerpark (McCormick) was plot 1 a 17 acre farm. So they were farmers. I looked at the Valuation Revision records on the PRONI website. They show John Mellon’s name remaining as tenant until 1871 when he was replaced by Charles Nicholl. I searched to see if there was a John Mellon death 1864 – 1878 in that registration area (Omagh). The only one I found was in 1864 aged 69. It’s not on-line free yet and you would need to pay to view it on the GRONI site, if interested. (Griffiths dates are sometimes out by a few years, and so it’s not unknown for someone who was dead to appear in Griffiths for some years afterwards). But on the face of it, the Deerpark Mellons were still in Irish land records 15 years after your family had gone to Scotland. So probably not the same John Mellon.
The main question is what did your John Mellon senior do? If he was a farmer, then the Deerpark entry could just possibly be him (if he had sublet for example and Griffiths didn’t bother to record it), but if he was a labourer or tradesman, then this probably isn’t him.
Griffiths isn’t as comprehensive as a full census. Not everyone was listed. Servants, people living with friends or relatives and those with houses of too poor a value to be listed (ie many labourers) are all excluded. And people who had left Ireland 4 years previously aren’t usually listed either.
