In my family tree, I have a Thomas Cowin (b. c.1782) of Lonan, married to a Margaret Kewley (b. c.1786) of Lonan, who had issue a Jane Cowin, born 1804.

Could there be a surname confusion between Kewley (which appears to be a common Manx name) and your Margaret?

Of interest, this Jane Cowin married, on 19/3/1831 in Lonan, a George Goldsmith (b 1805) from Lezayre.