I'm going on a cruise KEF-YUL, and my return flight is UA non-stop YUL-SFO. I am UA Star Gold and so I should have access to AC lounge at YUL, right?

PP lists 2 lounges at YUL:

1. Air France Lounge operated by Plaza Premium Group
05:00 - 22:00 daily
Note: Access may be restricted between 15:00 - 20:00 daily due to space constraints. (My flight departs at 18h30, so I probably will not gain access.)
Airside - International Concourse, near Gate 57. International flights only. (My flight is transborder, so I probably will not gain access. Double strikes!)

2. National Bank Lounge
04:00 - 22:00 daily
Note: During peak times, lounge access may be restricted based on capacity limits and lounge availability.
Airside - after Security Checks, the lounge is located near Gate 53. The lounge is not accessible to passengers departing on Canadian domestic or US Transborder flights and must hold a Boarding Pass with gates numbered 52 or up.

So it looks like I won't have access to either of the PP lounges at YUL. Hopefully the AC lounge is open to us.