It is family , perhaps lore, that has been passed down. I found her apparent arrival in NY July 7, 1846. I found Thomas, for the first time, in the 1847 and 1849 only Rochester directory. Her family is said to have been living there so it was thought to be St. Mary's. I looked all through the microfilm from St. Mary's for years before and after with no luck on the marriage.
I am hopeful that a baptism of one of her children will be in the microfilm coming but now maybe St. Mary's for their church is incorrect or maybe they just married in NY. That would be impossible to find.