Melt butter in deep skillet over medium high heat (or use slow cooker insert if it's stove-top safe). Sprinkle beef cubes with 2 teaspoons of the salt and 1 teaspoon of the pepper. Sear beef for about 2 minutes per side to form a crust; remove from pan and add into slow cooker.
To the slow cooker toss in the carrots, onion and potatoes. Sprinkle with remaining 1 teaspoon salt and 1 teaspoon pepper as well as the thyme and garlic.
In a small bowl, whisk together beef broth, tomato paste and Worcestershire sauce then pour into the slow cooker along with the Irish stout if using.
Heat on Low for 8 Hours of High for 4 Hours. 15 minutes before the stew is done cooking, stir in the peas. Serve warm.