Thanks for the helpful info hallmark,
But it is a complex story as I said at the beginning - By the 1901 and 1911 censuses, my McNabs were well and truly part of the Glasgow Scotland census!
The family (Francis McNab, Mary McHugh and 6 children) had in fact left Ireland, arriving in Glasgow Feb. 1864. Unfortunately, the four adult daughters all died within about 8 years, leaving 2 adult sons (one my gggrandfather) - But as we failed to find any Scottish death or census records for the parents after 1871, we have come to the conclusion that they could have returned to Ireland. They were said to come from Langfield (Drumquin C. Parish)... And we found a death for a Mary McNab wife of Francis 1885 "Mullinatoomog"... and for Francis McNab ("of Mullagharn") widower of Mary in Omagh Workhouse 1890. If these are our "parents".. it all adds to the solving of the puzzle which might one day "knock down our Irish brickwall"! What amazes me is that I have found so many McNabs in the Drumquin Catholic records... but not the McNabs we need!! So convinced in one way, that we are aiming in the right direction, but on the other hand, how come we cannot find them in the period pre 1864.. Could they have left the area, gone elsewhere in Ireland, before finally coming to Glasgow?
What a mystery!