Place the potatoes in a medium saucepan. Cover with water then heat over medium hear until potatoes can be easily pierced with a knife. Drain and mash. Mix in the butter, milk, salt and pepper. Set aside.
Meanwhile, cook the ground meat in a large skillet until no longer pink. Add in the onion, carrot and garlic, and cook for another 5 minutes.
Mix in the tomato paste and cornstarch until combined. Add in the Guinness and Worcestershire sauce.
Cover and reduce heat to low. Allow to simmer for around 10-15 minutes.
Preheat the oven to 375˚F.
Stir in the peas to the ground beef mixture. Pour this mixture into a 9 inch square pan.
Spread the mashed potatoes on top of the beef mixture.
Bake in the preheated oven for 20 minutes.