Hi Joker,
I see on the 1851 Census, Alexander 40 (born abt 1811) and Ann 36 (born about 1815) and their 9 children:
All listed as born Canada West: Mary 15, William 13, George 12, Alexander 8, Catherine 6, Richard 6, Sarah A 4, John 3 and James 1.
So Mary, was born around 1836 in Canada, and born when Ann was around 19-21 years old.
The names may at some stage be important as clues, as is the inclusion of a set of twins.
Here is an obituary for Ann, with lots of info (use search facility to search for "MARSHALL") :
http://www.glanbrookheritage.ca/deathsjanjun1895.htm* Died lat her home, Barton-on-the-Mountain.
* Had lived in Barton for nearly half a century (so since around 1845)
* Move to Barton from Ancaster.
* Came to Canada from Co. Fermanagh, Ireland in 1835 (so when she was about 18-19)
* Was living in Ancaster during the rebellion of 1837-38.
* Husband Alexander Marshall, was a loyalist volunteer during the rebellion.
* He served as sergeant commanding a squad of militia.
Cheers
AMBLY