I live near St. Johnston so I will ask about to see if there are any Magees/McGees still around. I had a look online for your great grandfather John McGee and also found his marriage details on rootsireland.ie married to Mary Harkin but in 1872. To see the actual record you have to pay credits.
I also did a search for their children and found 9 children with the last name spelt differently quite a bit.
- John McGee 1873,
- Thomas McGee 1875
- Mary Ann McGhee 1878
- Daniel Magee 1880
- William McGee 1881
- James Magee 1884
- Elizabeth Magee 1887
- Patrick McGee 1887
- Bernard Magee 1893
I assume that Elizabeth and Patrick were either twins or 'Irish twins' - I found all this info on rootsireland.ie which is a pay per view website so I only had the ability to search but not to view the records.
I also found this family living in the area of Monreagh when the 1901 Census was taken. http://tinyurl.com/cwpv6v6
And in the 1911 Census - http://tinyurl.com/cn75vhg
You may have already seen this info before but I will have another look around to see if I can find anything further back yet - I still haven't had a look at the other parts of your query yet.
I wish you all the best with your research.