There is an 1870 census listing for Margaret Hyland living in Paterson Ward 5, Passaic, NJ that lists her as having been born c 1825 in Ireland. She is the wife of Patrick Hyland and mother of Michael.
There is also a 1910 census listing for Margaret Hyland, married to Patric J Hyland, in Acquackanonk, Passaic, NJ. Here she is listed as 33 years old, which might be a bit young.
Finally, there is another listing of Margaret Hyland, b c 1867 in Illinois of Irish-born parents. She is a widow, living in Paterson Ward 2, Passaic, NJ in both the 1910 and 1920 census. Since your grandfather mentioned Margaret / Gretta only, and not a brother, this one might be your best bet.