Aha!!
Found John Sheridan in Greenlaw prison in 1847 - has to be a candidate for Thomas' father! Great thing is it says he was born in Co. Mayo, Ireland. Greenlaw was the prison for Berwickshire, which is the same county that Duns and Edrom are in. Looking good!
Prison entry for John says he was Catholic by the way. Not a surprise but good to remember pre 1855 BMDs on Scotlands people are split into OPRs which are Church of Scotland and the Catholic registers. These are by no means a complete set of records of birth, marriages and deaths in Scotland before 1855.
Bridget and John may well have married in Ireland but at least you now have a county to start with.
Emma