This small church and museum on the north side of Old Montreal is the on the site of the original church founded by Sister Marguerite Bourgeoys, one of the first settlers of Montreal, and the museum is dedicated to her life. The church is...just a church with interesting stained glass windows, stone altar, and the regular assortment of religious art, but it does have some interesting features with replicas of sailing ships adorning the chapel (it seems the chapel is a favorite with sailors). It's the views of Old Montreal and the Vieux-Port from the church tower make it worth the trek to this part of Old Montreal. The small museum is located in the crypt of the church and has an adjacent associated archeological site that you can also tour. The museum would be worth the admission to a someone interested in the history of a Canadian saint (and a woman to boot), but I can't comment on the archeological site since I didn't partake of it.
The chapel is thankfully free, but there is an $8 admission fee for the museum (it's steep considering all you get is the museum and tower access for the views). You can add the archeological site for an extra $2, which might make it more interesting (I don't really know). The closest metro stop is Champ De Mars, but it's still a hike up a really steep hill.
I say skip it if you need time for something else, but if you have the time (or the interest) then it's worth at least an hour's visit.