My greatx2 grandfather John Williamson was Irish being born in the north of Ireland c1820. He married my greatx2 grandmother Jane Marshall in 1855 in Scotland. His marriage certificate gives a lot of information and was obtained from Scotlands People at a small cost. If your John Williamson married in Scotland after 1855 then you should try to obtain his marriage certificate. It's well worth the charge. I found that mine was a widower with two living male children, whom I have been trying to find. There is a John Williamson on the 1851 Scottish Census with two sons and married to a Margaret living not too far away which may be the one.
You would do well to post all your queries on "Rootschat.co.uk" which is a FREE WEBSITE and with extremely helpful people.