Father Charles’s marriage:
https://www.irishgenealogy.ie/files/civil/marriage_returns/marriages_1860/09592/5478357.pdf
Marriage here in RAY (Errity)Presbyterian Church in Manorcunningham
Son and father farmers, Location Ruskey (Roosky)
Jane McKinney father John, location Knockbrack.

Samuel was in Roosky (another variant) at Griffith valuation) there was also a Samuel in neighbouring Tully. Attach a copy of the Griffiths valuation page which extracted at below.
https://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
Glencairn is a variant spelling of Glenkeeran, Area is close to Roosky and Knockbrack.
https://www.townlands.ie/donegal/raphoe-north/raphoe/magheraboy/glenkeeran/
https://www.townlands.ie/donegal/raphoe-north/raymoghy/kincraigy/roosky/
https://www.townlands.ie/donegal/raphoe-north/leck/magheraboy/knockbrack/
Presbyterians often buried in older burial grounds sometimes attached to old abbeys, as until early1800s they were not allowed separate graveyards, in these older graveyards often find that the details from early stones are no longer visible, from the death notice, it appears taken to an old family graveyard, these can often be some distance from residence. Some graveyards now have headstone photos online. I have not located any for your ancestors. I will search for older around Convoy.
Charles on 1901 census:
https://www.census.nationalarchives.ie/pages/1901/Donegal/Maghesaboy/Glenkeeran/1189188/
IrelandXO has a Meet and Greet program. If would like to receive the assistance of a volunteer when on trip to Ireland, complete below with date/best meet location:
https://www.irelandxo.com/meet-and-greet
If create a request and I am free I would be happy to meet.
Best Regards,
Marion