I think I should add that I have another mysterious Margaret Dinning (Dunning) who I had put on the back burner for a while. I have her in my tree as the daughter of Patrick Dunning and Sarah Docherty but I know she is not. Her marriage record shows that her parents were John Dunning and Mary Kanlay.
However, her father cannot be the same John Dunning who was the father of the Margaret Dunning married to Hugh McLean. This one was married to William Reid and according the census records, she was born about 1840 Ayrshire. The dates range from 1838-1845 but her age at the time of her marriage suggests 1837.
But here is the strange thing, I just noticed, as I haven't looked at her for a while that she was also in St. Quivox, Ayr on the 1861 census . She married there in 1857. Her son, Hugh, was born at 6 Smith's Lane, Greenock on the 28th February 1863. The very next entry is my great x 3 grandmother's daughter Helen born on the 5th March 1863 at 1 Highland Close. My great x 3 grandmother, Ellen McKenzie nee Dunning was living at number 7 Smith's Lane on the 1861 census with her sister Isabelle married to Andrew McFarlane.
It's a complicated puzzle indeed but I think this Margaret Dunning is as closely related as the other one.