It's beginning to look like your g-g-granny and mine, Mary McCurdy, were sisters.
On your side:
Marriage 15 February 1862 - Donald Black to JaneMcCurdy - witnesses Donald McKinley and John McCurdy;
Baptisms: 15 June 1862 - Mary Anne Black daughter to DonaldBlack and Jane McCurdy - sponsors John McCurdy and Isabella McCurdy
17 June 1866 - Daniel Black son to Daniel Black andJane McCurdy - sponsors Michael McCurdy and Jane McKinley
On my side we have:
20 August 1861 - John McQuilkan to MaryMcCurdy, in the presence of Donald Black(yours?) and Mary McCurdy (this may be the mother or aunt)
6 February 1975 - John MCCurdy to Isabella McCurdy (I have Belle's lines back quite far) in the presence of Dan McCormick and Mary McQuilkan [perhaps my Mary McCurdy under her married name?]
Similarly we have the following baptisms on my side for which the sponsors may be of interest:
1 June 1862 - John McQuilkin son of John McQuilkinand Mary McCurdy - sponsors James McQuilkin and Elizabeth McKinley
14 December 1863 - Susan McQuilkin daughter of JohnMcQuilkin and Mary McCurdy - sponsors James McQuilkin and ELizabethMcKinley
22 June 1865 - Mary Jane McQuilkin daughter of JohnMcQuilkin and Mary McCurdy - sponsors John McCurdy and ElizaMcCurdy
December 1867 - James McQuilkin son of JohnMcQuilkin and Mary McCurdy - sponsors James Doyle and KateMcQuilkin
16 April 1874 - Patrick McQuilkin son of John AllenMcQuilkin and Mary McCurdy - sponsors James McFaul and NancyAnderson
30 March 1878 [dob 27 March 1878] Joseph AllenMcQuilkin son of John McQuilkin and Mary McCurdy - sponsors James McFaul and AnnAnderson
Therefor, strong circumstantial evidence suggests that my Mary, your Jane and John (who married Belle McCurdy) were siblings. I further believe that the Father is an, as yet, unknown elder brother of a Ballycastle Archibald (b 1829 later moved to Port Glasgow) and Mary McCurdy (if unmarried, possibly aunt of John, Jane and Mary) whose parents were Gilbert McCurdy, a weaver in Carnmoon, and his wife Jane McCarroll.
This supposition is based on McCurdy cousins of my Grandfather, Patrick Kelly, in Port Glasgow and Greenock, some who still carry the very uncommon forenames, Gilbert and Alexander (Belle McCurdy's Dad's name).
Alexander McCurdy married Peggy Weir and had five children: Donald; Angus; John; Belle; Rose. The children of Donald were: Alexander; Donald; Agnes; John; Neil; Mary; Betty; Margaret. The first three died unmarried. Neil went to New York and married Kate Gallagher and had four children: Edward; Daniel; Mary; Margaret. Mary married a McKinley and they had five children: Patrick; Alexander is dead; Daniel; James; John. All born on Rathlin.
Belle McCurdy - d/o Alexander and Peggy (Weir) married John McCurdy and they had two children: Esther and John. Esther married Joseph McQuilkin son of John Allan and Mary McCurdy.