I sent through what I found on 1851 census and it got away from me before I could give you the 1861 census.
Re 1851 census. The Susan mentioned is the only one I found on 1851 census anywhere.
1861 census Matilda Dundas County
James, Ireland, 65
Marjory, Ireland, 55
John, Canada, 23
Wm, Canada, 18
Francis, Canada, 14
I cannot find Susan on 1861 census under either name.
How did Susan get to England?
On 1911 census re George it says he is an 'army pensioner' but all other census list him as labourer.
Was he in Canada with British Army and met Susan?
First child was born England c1869. Unfortunately none of the census give an immigration date for Susan.
Look on this site for her death and get the registration which should have her parents on it. She is still alive on 1911 census. http://freebmd.rootsweb.com/cgi/search.pl