Sharon,
You mention that you ordered the certificate. Just for information, if you want, you can view the originals on-line. That only costs ?2 and so is cheaper and quicker than ordering a copy.
Irvinestown/Lowtherstown is where the civil registrar for that area is located. So all marriages, births & deaths in that administrative area get registered there and appear under that town on the Irish civil indexes on sites like familysearch. So when I said that the marriage was registered there, what I meant was that the marriage is listed under that town in official records, as will be any births and deaths. Something you may find handy if looking for other members of the family.
The marriage certificate shows Thomas Gibson, 20, bachelor, labourer, of Aghaleague, s/o Abraham Gibson, farmer; he married Bessy Graham, full age (ie 21 or over), spinster, also of Aghaleague (there?s a ditto). Now her father?s name is also shown as a ditto, which is obviously nonsense. So that must be a mistake. Her father?s occupation was farmer. The witnesses were Robert Simpson & Andrew Knox. (Several Simpsons lived in Aghaleague).
Aghaleague is 392 acres in size, and in the 1901 census there were 74 residents in 14 houses. It?s a rural farming area about a quarter of a mile south of Ederney village up a minor (unnamed and unnumbered) back road. Griffiths Valuation for the townland in 1862 shows 4 Graham households (all of whom are probably related to each other). John Graham was farming on plot 1, a farm, outbuildings and 10 acres, Alexander Graham with a farm, outbuildings and 15 acres on plots 18 & 19, William Graham with a farm, outbuildings and 19 acres on plots 20 & 21 and George Graham with a house on 31b. (These would all be easy enough to find today, provided you had a townland map). There were no Gibson households listed in Aghaleague in 1862.
http://www.askaboutireland.ie/griffith-valuation/index.xml
The revaluation records show George Graham?s name deleted for 31b in 1886 (replaced by John Livingstone). That would suggest he died around then. Griffiths clerks were often out by a few years. There are just 2 George Graham deaths registered in Irvinestown in the period 1880 ? 1901. Both died in 1881. One had an estimated year of birth of 1791, the other 1805. You might want to look them up to see if either was your George from Aghaleague.
http://www.proni.gov.uk/index/search_the_archives/val12b.htm
There was one Graham family still in Aghaleague in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Fermanagh/Magheraculmoney/Aghaleague/1354663/
and in 1911:
http://www.census.nationalarchives.ie/pages/1911/Fermanagh/Magheraclumoney/Aghaleague/520704/
The revaluation records indicate this family acquired plot 7 in 1889. Mary replaced Robert around 1910 in Griffiths, and her name was still against the property in 1929. Looking at plot 20/21, I notice that William Graham, was replaced by Robert Graham in 1885, and then he gave that farm up in 1889. (Presumably to move to plot 7). So William is probably Robert?s father.
So there was a George Graham living in Aghaleague in the 1860s, which fits with your existing information. Your next step is probably to search the Magherculmoney COI baptism and marriage registers for Bessy?s baptism in the 1830s, and that of any siblings. That should hopefully establish her parents names. Magheraculmoney baptisms start in 1767, as do the marriages. There is however a gap in the marriages 1818-1825. The records are not on-line anywhere, so far as I know, but there is a copy on microfilm in PRONI, in Belfast. (A personal visit is normally required to view them).
NB Church of Ireland is the same as Church of England. So they were Anglicans/Episcopalians.
Elwyn