So I rewired as DLane suggested. I just put a receptacle into the feed that went to the LED ceiling light. What surprised me is that they had actually run it as 240V! Good thing I checked the junction box before plugging anything in. After switching it over to 120V the ceiling light still works (so I guess it accepts 120-240V) and I now have a (hopefully) reliable 120V receptacle for the irrigation controller.
Now to get a replacement irrigation controller …
