Bring a large cast iron skillet to medium-high heat. Add 1 tablespoon butter and let melt; swirl to coat the pan.
Add garlic and onions. Move around pan until garlic is fragrant and onion softens. Add another tablespoon butter and let melt. Add potatoes, salt and pepper. Stir to combine.
Keep heat on approx. medium and fairly constantly move potatoes around the pan with a small spatula as they cook. Once potatoes are firm but soft, approx. 10-15 minutes, add corned beef.
Since the corned beef is already cooked we only need to move it around the pan a few minutes to start warming up. Then cover and remove from heat.
Prepare your poached eggs.
Divide corned beef and hash between plates and serve with a poached egg on top with fresh cracked pepper.