I have searched for her off and on for many years and I thought I would give it one last shot.
Margaret Mckay m John Jarvie (Jarvie/Gentles line) in Blythswood, in 1870. Her marriage and subsequent documents suggests she was b 1838 +/- 1 year, but I have looked up to +/- 10 years.
Prior to her marriage she was a domestic for James Smith a stockbroker at 7 Park Circus. This is the only record I can find of her prior to her marriage as it is also the address given on her marriage registration.
This couple had 3 children (Ann, Walter, Margaret), all of her children and her husband predeceased her. On her death in 1913 her widowed daughter-in-law was the informant. And her death supports her marriage and the census info as they relate to her origins.
Her father Duncan has listed as a Road Contractor yet I find no Duncan in the Scottish or English Census to support this. I suspect he was more likely a labourer who worked on the roads but that is just my speculation. There is a Duncan/Ann in Sutherland with a daughter Margaret but this is not her as this Margaret married locally.
Margaret's birthplace is listed as Kilbride, Lochgilphead, Dalavaich, Oban and Dunach. There is an 1851 Census entry in Inveraray for a Margaret a Domestic born in Dalavaich but I am not convinced it is her due to the commonality of her name and occupation so she remains a possibility.
I also looked for a Margaret being the illegitimate daughter of Ann Mckinnon but no luck pursuing that scenario either.
Don