You have probably seen the three Margaret Nelson bapts in the IoM on the IGI within 5 years of 1840? [FamilySearch.org]. There is no way of knowing whether one could be your Margaret without more information to be able to look for her in censuses, and to see her place of birth given in those.
I haven't found a Margaret Keough of that age born Isle of Man in English/IOM censuses.

Sue