The stew requires about four hours cooking time. I always drain and rinse canned beans because the liquid is usually salty. Canned beans are already cooked, of course, but you want them to absorb some of the flavors of the lamb stew without falling apart.
Beans stay quite firm if a recipe contains an acidic ingredient, such as tomatoes.
So you could add the beans about 30 minutes before the stew is done, they'll absorb some of the flavor, and shouldn't get mushy because of the acid from the tomatoes. If a dish contains no acidic ingredients, add canned beans during the last 10 minutes of cooking time.