There is an admirals club, but it's not in the same concourse as the YUL flights, you'll need to screen security twice (unless something recently changed). There is a nice US lounge before security next to the YUL concourse. They also have hours, check online.
Now, after searching for a while I saw that
1) AA LGA-YUL is going from terminal B concourse C.
2) The US club is in terminal C.
3) The Admirals Club is in terminal B but in concourse D after security.
So I don't see any way that I'll be able to use any of them
Please CMIIR