Statutory birth registration only started in Ireland in 1864, so you won’t find a birth certificate for either John or Sarah. Prior to that you need to rely on church baptism records, where they exist. So you need to know the couples denominations and townlands.
James Wilson’s marriage to Jane Kennedy in 1850 is on-line. 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. If you provide details of their townlands (addresses), occupations and fathers names, I’ll advise what other records may be available
Tradition was to marry in the bride’s church which should be on the certificate (unless it was a Registry Office marriage). That church may be the place to look for her baptism and that of any siblings. So let me know where the ceremony took place.
I looked for Benjamin Johnston’s marriage to Mary Collins but did not find it in the years 1845 – 159. Possibly it was before the start of statutory marriage registration.
I do see a Benjamin Johnston living in Derrykerran townland, outside Portadown, in Griffiths Valuation for 1864. He had a 1 acre farm on plot 24 there.
Here he is in the 1901 census. It would appear that one of his daughters married a Holland and lived in Scotland. Should be easy enough to trace if it proves to be the right family.
http://www.census.nationalarchives.ie/pages/1901/Armagh/Drumcree/Derrykerran/1031340/
He died on 30.3.1903 aged 82. He was a widower.
