First Time in Old Montreal
Here are a couple places we like that will serve you a late dinner in Old Montreal:
Barroco: trendy, intimate Spanish restaurant.
BEVO Bar + Pizzeria: happening pizzeria, touristy area (1st pic below: Saccense Pizza).
Mechant Boeuf: happening gastro-pub.
Weekend lunch is trickier because most places only serve brunch on weekends, but you can try Verses in Hotel Nelligan. They actually have a full lunch menu on the weekends.
Club Chasse et Pêche and 400 Coups are probably the best restaurants in the area, and among some of the best in the city. I should mention the old standard, Le Toque, to me a different animal. Probably the closest to a Michelin starred/white cloth restaurant you’ll find in Montreal.
Slightly less expensive, Le Local is also fun.
Da Emma or Graziella for Italian, and Helena for Portuguese are also options.
I answered without price as a concern, since you didn’t mention it.
For lunch on a Saturday, that is much harder. Old Montreal empties out during week-end, you’ll find mostly tourists (as compared to during the work week). Look at Olive & Gourmando and Le Cartet, both very good, but menu is probably more brunch than lunch on Saturday. For the Saturday at least, if you can get out of the neighbourhood, you’ll enjoy it more.