John was born in Ireland around 1847. He came to the US in 1851. He married Mary in 1965.
John is not listed on the 1880 US Census but shows up again on the 1880 US Census. Mary is not listed as a widow on the 1880 Census - she is listed as Mrs. John Walker.
The 1860 US Census has John Walker living with James and Mary Walker who appear to be his parents. Based on this, his parents were in the US in 1846 as John was born in Ireland and then his brother William in PA.
James Walker M 53 Ireland
Mary Walker F 47 Ireland
Thomas Walker M 24 Ireland
John Walker M 16 Ireland
Wm Walker M 14 Pennsylvania
Patrick Walker M 12 Pennsylvania
Mary A Walker F 10 Pennsylvania
Margaret Walker F 3 Pennsylvania
United States Census, 1870," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MZPP-YHB : 17 October 2014), John Walker, Pennsylvania, United States; citing p. 9, family 65, NARA microfilm publication M593 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 552,863.
United States Census, 1880," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MWFL-NVK : 10 August 2016), John Walker, Carbondale, Lackawanna, Pennsylvania, United States; citing enumeration district ED 26, sheet 69B, NARA microfilm publication T9 (Washington D.C.: National Archives and Records Administration, n.d.), roll 1137; FHL microfilm 1,255,137.