Fionah,
Thinking about John Black’s death again, it occurred to me that the 1871 census should record Elizabeth’s marital status. Should be “Mar” is she was married and “W” or “Wid” if her husband was dead. What does it say? That way I will know which side of 1871 to search for John’s death.
Knowing that Elizabeth’s maiden name was Maxwell is helpful but we still face the problem that not all churches in Donegal have records for the 1820s and 1830s when they probably married.
I searched the 1901 census of Co Donegal and there were 140 people named Maxwell. 4 were RC, 16 were Presbyterian and the remainder were Church of Ireland. So if you don’t know the family denomination, then statistically they seem likely to have been Church of Ireland.
Tradition was to marry in the bride’s church and then she would attend her husband’s church. So marriage and subsequent baptism of children are not always in the same church.
I looked at the surname Black in Donegal. There were 135 in the 1901 census. Every single one was Presbyterian. So my guess here is that Elizabeth may have married in the Church of Ireland and their children would then be baptised in a Presbyterian church.
I looked for Elizabeth’s death in Scotland (now knowing that her maiden name was Maxwell). I did not find it. So I wondered if she had died in Ireland. I searched the deaths 1871 – 1901 and found 1 that might be your lady:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1881/06430/4847557.pdf
She died 15.8.1881 at Lisnaree townland in Stranorlar’s registration area. She was 86 (which is about right) and was the widow of John Black labourer. The informant was Martha Black of Lisnaree.
I looked at Griffiths Valuation for Lisnaree in 1857 and there is a John Black listed there. He had a labourers cottage and a garden of 25 perches (40 perches in a rood, and 4 roods in an acre). That was on William Finlay’s farm. Today that’s on the R236 road between Convoy and Stranorlar. There were no Blacks listed there in 1901.
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
So if I have found the right family, then the local Church of Ireland churches in that area are: Convoy (marriages from 1845, baptisms from 1871. Earlier records lost in the 1922 fire in Dublin). Kilteevogue, (baptisms from 1818 and marriages from 1845), and Stranorlar, (baptisms from 1802 and marriages from 1821).
So if you are looking for the couple’s marriage, the only 1 of the 3 with any records is Stranorlar. Might be worth checking it.
For local Presbyterian records (childrens baptisms, and perhaps the couples marriage) Convoy has baptisms from 1822, marriages from 1845; Stranorlar has baptisms from 1821 and marriages from 1846.
There are also two Reformed Presbyterian churches in the area (sometimes known as Covenanters) but neither has any records for the period you need.
You could try rootsireland who have some Portestant records on their site (subscription) but I suspec you are not going to find most of the above records on-line. However there are copies, for both denominations, in PRONI (the public record office) in Belfast. They are not on-line there and you either need to go in person or get a researcher to do that for you.
Incidentally if the family was Presbyterian and lived in Donegal that indicates they are most probably descended from Scots who settled there in the 1600s. But possibly you knew that anyway.
Let me know about Elizabeth’s marital status in 1871 and I’ll have another look for John’s death. I do now see one in 1870 aged 78, in the Stranorlar area, which could be him. But it’s not viewable on line free yet, so I’d like to know whether he was likely to be alive in 1870 before suggesting you order a copy of it.