His sister's name from the manifest from the S.S. Teutonic looks like Maria.
Some basic data would help. When were George and Mary born and when and where did they die? Do you have when and where they married? What were their children's names? The more you post the easier it is to help you. Start with what you are certain of and go from there.
BTW there was a George R. Gannon born ca 1880 listed in the New York censuses, but he was born in NY and not Ireland.