Peter,
I had a look at the 1857 marriage record. It doesn’t tell us much about the couple. The witnesses were Thomas Sherry and Margaret Donnelly, but there are no addresses or occupations. Tradition was to marry in the bride’s church so presumably Margaret came from the parish of Armagh. Probably Edward did too but we can’t be absolutely certain about that.
Do you have any occupations for Edward or Margaret’s fathers?
Death registration started in Ireland in 1864. I searched the post 1864 records for an Edward Slevin of roughly the right age to be the father of Edward born c 1831. He died 7.10.1873, aged 58. So born c 1815. That death certificate isn’t on-line free yet. You can view the original certificate on-line on the GRONI website, using the “search registrations” option:
https://geni.nidirect.gov.uk
You will need to open an account and buy some credits. It costs £2.50 (sterling) to a view a certificate.
I also looked for death for Margaret Slavin, 1864 onwards, again in the Armagh registration area. I only found 1:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1880/06491/4868082.pdf
She died 11.2.1880 aged 60 and was the widow of a shoemaker. The informant was Joseph Slevin. Both lived in Castle St, Armagh.
I also found this death of a Margaret Slavin, in 1899, wife of James a clothes dealer. I wonder if that was the same James who informed on the 1880 death?
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1899/05799/4640670.pdf
James in 1901:
http://www.census.nationalarchives.ie/pages/1901/Armagh/South_Ward/Thomas_Street/1011218/
I wouldn't pay too much attention to the ages on the death certificates. They were usually just a guess and could often be out by 100 years or more, for someone in their 60s or older. Most people in Ireland in the 1800s didn't celebrate birthdays and often had little idea of how old they were.
I searched the street directory for Armagh up to the mid 1870s, but did not find any reference to Edward Slevin. (Not everyone was listed though).
I had a look for Edward & Margret’s baptisms but did not find either. Nor could I find their parents marriages. I couldn’t see a death for any Francis McBride around the Armagh area 1864 to 1901. The only possible Margaret McBride died in 1876 aged 50. Again slightly too young but the age could be wrong. You might want to look it up to see if you get any clues.
That apart, I am afraid I have drawn a blank. I can’t find any reference to either family.