Wow! Loughguile I think this is a break through!!!! Well done.
The John McGill aged 77 in 1911 with wife Margaret would seem to be my great great grandparents. I know they had two children: Patrick and James (my great grandfather) who both emigrated to Durham England and died in the 1890s which would account for the six children, 4 living; but I don't know of any other children born. At the time of Patrick's marriage in 1890 John was a labourer, I guess he could have changed his occupation.
The Mary Ann you listed as age 6 in 1911 is not my grandmother. She would have been 15-16 years old and of course born in Durham England. Her father James died in Durham in 1899.
Where is Lavin Lower Castlequarter?
Do you know whether the 1901 census for Ireland is available yet?
Best wishes
Amanda