Be warned that this place has no public restroom, and very little in the way of facilities for any visitors, even parishioners (there's no drinking fountain, for instance). When I asked about this, the man who I talked to (apparently a janitor) was incredibly condescending and unwelcoming, something that does not really befit a Christian church when you think about it.
I would recommend skipping this church unless you are dying to see St. Peter's Church in Rome and can't afford the airfare to see the real thing. Otherwise, I would go to St. Joseph's Oratory elsewhere in Montreal, which is also a working cathedral based on a European model, but is much more welcoming to visitors. The church interior is very beautiful, but rude and unwelcoming people can make anything unattractive. Thanks for reading, and enjoy your stay in Montreal.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC