CFD,
McGee Island is normally known in Ireland as Islandmagee. (That may help if you are researching the area). It’s not actually an island though reportedly it was at some time in the long distant past when sea levels were higher. It’s a peninsula in Co Antrim near Larne. About 25 miles from Belfast.
https://uk.images.search.yahoo.com/search/images;_ylt=AwrJQ54rvCZevVAAARFLBQx.;_ylu=X3oDMTBybDA1bGNhBGNvbG8DaXIyBHBvcwMyBHZ0aWQDBHNlYwNzYw--?p=Islandmagee&fr=yfp-t
None of the churches on Islandmagee has any records for the 1700s. The earliest is the 1st Presbyterian church whose records start in 1829. So there’s unlikely to be any documentary record of Mary Hill, if that’s where she originated. Looking at Griffiths Valuation for Islandmagee in 1861, there were about 40 Hill households there then, so the name is prevalent in the area.
If William Cunningham comes from Ballymoney in Donegal then that’s in the parish of Burt. You have a similar problem with lack of records there. None of the churches in the area has any records for the 1700s making it very difficult to trace him. I checked Griffiths Valuation for that townland but there were no Cunninghams listed in the 1850s. There are a number of places named Ballymoney in Ireland, including one in Co. Antrim (not too far from Islandmagee). Are you sure yours came from Donegal?