1871 census - Margaret Burden 17 b.Ireland, servant in Peel to Mary Ann Roberts,shopkeeper.
From the information that has been supplied - 1881 Margaret McCall was born about 1856, therefore she couldn't be the wife of William in 1867. I believe that the original wife, Margaret, died 30 Dec 1879, as stated after her son Archibald was born 29 Dec 1879. Before the date of the 1881 census William married Margaret Burden/Burdin.
William's first two known children were born in Scotland, Mary Ann about 1873 and Elizabeth about 1875, therefore he must have moved to Peel between 1875 and the birth of Archibald Dec 1879.
Maybe the first Margaret was a Margaret Robinson and not Margaret Marr?? There is a William and Margaret McCall both born Scotland, living in London at this time, perhaps this Margaret was a Marr!