Hi Jane
I see the Col has replied to this in some way HERE
I couldn't see any Campbell-Francis marriage for Longford or surrounding areas despite the fact that many parishes in Longford have surviving marriage records from the 1820s and 1830s, perhaps they were married in a parish where did not start until later on. I see that Elizabeth's surname is not recorded on Robert's baptism but he was baptised in Killaloe as you know on 4th August 1825 (born 3rd July 1825).
I think that the best thing you can do is start with Robert in Ontario and find out his birth/baptism details and if possible those of his parents and their siblings. In this way, sponsors/godparents and witnesses at weddings will become known to you and any with a Francis-Campbell with point you in the direction of a family connection. Don't forget to include a search of local newspapers or obituaries, sometimes the latter for example, list the names and those attending funerals and their connection to the deceased. This is an easier process that trying to make a connection through Elizabeth/William - start with what you know and work back.
Best wishes
Clare
Genealogy Support