George,
I can’t see the marriage in the RC records. I do see the baptism of William Henry McDermott to those parents in October 1852 (sponsors Roger McDermott & Mary Maxwell).
https://registers.nli.ie/registers/vtls000633058#page/11/mode/1up
There was also a Georgina McDermott on 20.5.1850 to parents George McDermott and Ann Bell, sponsor Sally McLaughlin.
https://registers.nli.ie/registers/vtls000633057#page/55/mode/1up
Both events at St Columbs RC Derry. St Columb’s baptisms start in 1823 and stop in 1826. There’s a gap till 1836. Possibly George & Ann were born in the gap. The 1848 marriage records are complete but there’s no sign of one for the couple in 1848. There were only 16 marriages in the chapel in 1848 so fairly easy to double check. Tradition was to marry in the bride’s church. Perhaps they married in a church whose records do not cover 1848?
Your George was a tailor. 1852 Street Directory for Londonderry lists a John McDermott, clothes broker, in Cowbog St, and R. McDermott, tailor, in Richmond St. I wonder if the R is the Roger who was a sponsor to William Henry?
https://www.lennonwylie.co.uk/1852PD3.htm
Here’s a John McDermott, tailor, who died in Derry. The occupation suggests there might be a connection:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_retu…
If George & Ann both died in Scotland, you should get their parents names from the death certificates.
1831 census of Derry lists 4 Buchanan/Buchannan households. George in Ferry Key St (now Ferryquay), Miss B in the Fishmarket, William in Springtown & John in Ferry Key St. They all look to have been Presbyterian so maybe Ann was baptised Presbyterian? It might be worth checking 1st Derry Presbyterian’s baptism records for her. They start in 1815 and there’s a copy in PRONI in Belfast.
http://www.census.nationalarchives.ie/pages/1831/_/_/Templemore/Ferry_Key_St/112/
Buchanan + Presbyterian + birth in Derry tends to point to Scottish origins for that family. The Buchanans probably arrived in the area from Scotland in the 1600s as part of the Plantation.
There are about 20 McDermot(t) households in the City of Derry in the 1831 census. (Parish is Templemore). All but 1 were RC. None in Cowbog St or Richmond St. If you get George’s parents names from his death certificate, you may be able to locate his family in the census.