From the 1885 marriage cert - who were his parents?
You would need to search SP for Daniel McMillan marriages and match the entries to his parents
It sounds as though he was a mariner and there is an 1881 entry for a Daniel McMillan b 1855 Saltcoats Ayr - a little younger I know - he was moored in Lisbon aboard the Iberia on census night
The 1861 census shows his mother as Margaret