Family came to Canada before 1856, as evidenced by daughter’s birth year of 1856 in Ontario.

1871 Canada Census, Wellington north, Arthur Township, p96/119
McMahon, John, age 68, farmer, born in Ireland
Sarah, age 65, born in Ireland
John, age 30, born in Ireland
Lawrence, age 28, born in Ireland
Bridget, age 24, born in Ireland
Sarah, age 15, born in Ontario
David, age 12, born in Ontario