Griffiths Valuation shows no Frank (Francis) Leonards in Co. Armagh (1848-1864). There are several in Co. Fermanagh while others are in the south. I also cannot find a marriage for them in Ireland in the Irish Civil Registrations, so perhaps they were married in Scotland?
The 1860 census shows another child, Frank, born after Roseanna, with those two children and the parents, all born in Ireland. Mary's birth place is given as Illinois and Margaret Jr. born in Iowa.