What is your source that refers to Montreal County? Maybe if you could give us the context of what you are looking for, someone could help direct you appropriately.
I noticed this very morning, that the LDS Family History Centre states its Montreal address as being in Montreal, "Montreal County", Québec, Canada.
This very genforum not too long ago referred to Canadian provinces as "states" and subdivisions as "counties", which is not consistent with the Canadian model of political divisions.
There is no Montreal county as such in the province of Quebec.
There is the City of Montreal, the Montreal Urban Community, the island of Montreal, and there is the judicial district of Montreal.Then there is the diocese of Montreal.The island of Montreal is divided into "electoral counties " that serve for the election of members of the national assembly, and members of parliament, but none of them with the name of Montreal by itself.