I searched the Irish Family History Foundation's database for Louth. Rcds by your DOB are generally complete.
For Michael McGuinness, B 1857 +/- 4 yrs, I found 3 B rcds. One of these is in Dundalk. The forename is spelled "Michl" in the database. There was a priest at the time who wrote abbreviated forenames often. The other two rcds are in Drogheda and Louth parishes. One of the rcds MAY be for your ancestor. I rate your chances at abt 80%.
Your msg sounded as if you were about ready to give up the search. Google the Foundation's name to lead you to the database. Post a msg to Genforum to tell me your search results.