Ok let me recheck some things.. starting from her birth
Ireland, Births and Baptisms, 1620-1911 about Margaret Mitchell
Name: Margaret Mitchell
Gender: Female
Birth Date: 12 Mar 1872
Birth Place: Claudy, Derry, Ireland
Baptism Place: Claudy, Derry, Ireland
Father's Name: Samuel Mitchell
Mother's Name: Elizabeth Glenn
FHL Film Number: 255841

If she arrived in 1890.. there are only two born close to her birth year .. both arriving in NY
New York, State Census, 1892 about Margaret Mitchell
Name: Margaret Mitchell
Birth Year: abt 1871
Birth Place: Ireland
Age: 21
Residence Place: Rochester, Monroe
Ward: Ward 11
Election District: 03

New York, State Census, 1892 about Margaret Mitchell
Name: Margaret Mitchell
Birth Year: abt 1873
Birth Place: Ireland
Age: 19
Gender: Female
Citizenship: Citizen
Residence Place: Brooklyn, Kings
Ward: Ward 17
Election District: 15