Place the celery, onion, carrots, and garlic in a large slow cooker. Season the lamb with salt nd pepper and nestle into the slow cooker over the vegetables.
Add the smoked paprika and oregano. Pour in the tomatoes, beer, and broth. Cover and allow to cook for 5 to 6 hours on low.
Carefully remove the lamb and shred, discarding bones and any fat. Return to the crock pot.
Add the potatoes to the crock pot and cook for another 2 to 3 hours over low, until the potatoes are fork tender.
In a small bowl, whisk the water and cornstarch together to create a slurry. Stir into the crock pot and allow the stew to thicken, 30 to 40 minutes. Add the peas and heat through.
Ladle into warmed bowls and garnish with fresh parsley.
You can prep this recipe a day ahead, up until you add the potatoes. Allow the soup to cool, if making ahead, spooning any fat off from the surface and discard. This stew is perfect in a Dutch oven or other stock pot as well if you do not want to make it in the crock pot. Just simmer over low.