This is called the FAN Method: Friends, Associates, and Neighbors.
In the 1855 New York State Census (images available at FamilySearch.org), George W Duryea (1823-1864) and his family are living in New York City, 19th Ward. With them is a servant, Mary Walpole, age 28, born in Ireland. This Irish servant later married George's brother, Jacob Duryea (1824-1899). This shows us how Mary Walpole probably crossed paths with Jacob Duryea.
Mary died in 1915 in Jersey City. She was buried at Hoboken Cemetery in North Bergen, Hudson County, New Jersey.
|Unlike other sources, Mary's death certificate gives her place of birth as England, not Ireland.|
Her parents are listed as John Wallpole and Mary Brazil.