Kathleen-- I recommend paying the Centre; the records of civil marriage are available for 1870; it is a lot easier, and cheaper, than flying to Ireland! Michael and Anna came out to Minnesota because Anna's sister was already here. Under the free immigration program in the early 1880's they could go only to the US if there was a family which offered to find them employment. Please let me know what you find out. I know a fair amount about Anna Dixon's family in Ireland and the US but I know nothing about Patrick's family.An only child seems unlikely in that time.