Mary was born sometime between 1814 and 1817 in Ireland to Patrick and Honora Walsh.

Mary and Dennis' eldest daughter, Margaret, was born in Ireland about 1849.

Mary and Dennis' son, Michael Dunn, was born about 1852 in the District of Columbia.

Mary and Dennis' daughter, Ellen Catherine Dunn, was born September 14, 1856.

Mary and Dennis' youngest child, William, was probably born in September or October of 1859.

Mary and her husband, Dennis were enumerated in Ward 4, Washington D.C., Tennellytown post office. Mary notes that she is 34 years old, while Dennis is aged 35 and employed as an Irish laborer. The couple lists their personal property value at $550.00 and valued their real estate holdings at $75.00. Neither party can read or write. 11-year-old Margaret, born in Ireland, is attending school with her 9-year-old brother Michael, also born in Ireland. The family lists 4-year-old Ellen as having been born in Maryland, and 10-month-old William as born in D.C. Also living in the couple's household are three borders: 37-year-old Bridget Milliff, an Irish born cook, 28-year-old John Milliff, an Irish laborer, and little 5-month-old George Milliff, who was born in Washington D.C.

Mary, two of her children and a family member were enumerated in Georgetown in the household of 28-year-old Daniel Kearnum, an Irish born common laborer, and his 23-year-old wife, Margaret. Mary Dunn, now a widow, reported that she was 53 years old and born in Ireland. She states that she cannot read or write. Her daughter, 13-year-old Ella Dunn, was born in Washington D.C. and is currently attending school. Mary's son Michael Dunn is listed as being 18 years old, born in Washington D.C. and employed as a common laborer. Neither of her children can read or write. Also enumerated in the household is 12-year-old James Welsh, born in Missouri to foreign born parents. Like Ella, James is currently attending school and cannot currently read or write. Mary identifies her relationship to James as "one of family".

According to Boyd’s 1871 Directory, Mary Dunn, widow of Dennis Dunn, resides at 1000 10th se near the naval yard.