John McCann is an extremely common name in Ireland. By way of example there were 92 people of that exact name in Co. Armagh in the 1901 census. So we need a bit more information if we are to trace this particular one. Do you have his parents names from his marriage or death certificate? What denomination was he? Do you have any other information about where in Armagh he may have lived? What were his and his father’s occupations?