Edit: Sorry typo in the title, Mary not marry
It's not marry, its Mary, and the the response to that is Dia is Muire is Pádraig duit, which adds in St. Patrick.
You may be interested in this AskHistorians thread from 2013: "Dia dhuit" meaning 'God be with you' is the Irish word for hello, how did Irish people greet each other before Christianity arrived?
The general consensus seems to be "we just don't know."