Preheat oven to 375°F.
In a large sauté pan over medium-high heat, heat the oil, then add the onion, carrot, and meat. Cook until browned, 8 to 10 minutes.
Drain the fat and add the Irish stout, tomato paste, and herbs. Simmer until the juices thicken, about 10 minutes, then add the peas.
Pour the mixture into a 1 ½-quart baking dish; set aside.
Meanwhile, bring the potatoes to a boil in salted water. Cook until tender, about 20 minutes; drain.
Mash the potatoes with the butter, milk, and salt.
Spread the potatoes over the meat mixture, then crosshatch the top with a fork.
Bake until golden, 30 to 35 minutes.
