Given there weren't any obvious baptisms in either Peebles or Stirling for Andrew, I decided to look up Mary's marriage to see if that gave any leads.
The 1827 marriage of Mary FAIRBAIRN to John DARLING was actually in Fogo with "both of this parish" (I didn't check whether or not there were also banns in Duns). I noted that two entries prior was that of an Isabella FAIRBAIRN marrying an Alexander MILLER, again both of this parish.
Whereas Mary's baptism isn't obvious in Fogo (nor any for her brother Andrew), Isabella's was likely to be that to James, smith at Whinkerstones, who married Margaret Hog in 1795 at Fogo (by Mr Davidson Minister of the Burgher Congregation in Duns) There is also a daughter Margaret baptised 1798 at Fogo, again to James, smith at Whinkerstones. And no further baptisms in Fogo that I noticed to James.
This may or may not be a complete red herring for Andrew and family, but just in case it helps anyone I thought I'd post it.
Re the Robert of Bowmanville part of Gayle's query.I believe he belongs to Sanna's line of John FAIRBAIRN and Elizabeth YULE.I don't know of any connection, Sanna might.
Male FAIRBAIRN descendants of any of these, or other lines, most welcome to come along and represent their line in the dna project, which now has full results in from 13 matching Borders FAIRBAIRN family representatives (plus 4 others in separate lines), and needs more representatives to help piece together this jigsaw with all its missing pieces.