Hi Mike, answering queries is not quick like when I was young, have been asked to look at unanswered queries on the site and since you posted civil records from 1864 are free to view on this site, you must sign in but there are a lot of Michael Mannions getting married in the period I searched, assume the civil district is Clifden but these names change on the register over time, also probably married in the brides parish.
You will need his fathers name to be sure of the correct records also but if you find the birth certificate you should be ok but may find it on his death cert in the USA. https://civilrecords.irishgenealogy.ie/churchrecords/civil-search.jsp Church records on this site are not for Galway.
Good Luck
Pat