Preheat oven to 400f and set one rack in the middle and one towards the top 1/3 of the oven.
While the potatoes are boiling make the filling. Heat a large heavy pan or pot to medium-high heat then add the lamb.
Cook until well browned (about 7-10 minutes), then break up the lamb with a wooden spoon or meat masher. Next, add the carrots, onions, and celery along with a pinch of salt. Saute until the veggies are soft (about 5-7 minutes) then add the garlic and cook for another 1-2 minutes or until fragrant.
Add the tomato paste, stir, and cook for 3 minutes. Next, add the Guinness and turn the heat to high. With a wooden spoon scrape the bottom of the pot to remove all of the brown bits. Cook until the liquid has almost completely evaporated (about 3-4 minutes).
Turn the heat down to medium and add the flour. Cook for 1-2 minutes or until there is no white flour remaining in the pan. Add the beef stock, thyme, rosemary, peas, and Worcestershire sauce. Bring to a boil. Once boiling, lower the heat to a simmer and cook until the liquid has reduced by half.
Taste test the filling and adjust salt, pepper, thyme, or rosemary if required.
Add the filling to a 9x13" baking dish. Spread the mashed potatoes on the top and with a fork or spoon fluff the potatoes making mounds or points. Sprinkle the remaining 1/4 cup of grated parmesan.
Bake for 20 minutes in the center of the oven. Move the dish towards the top of the oven and broil for 1-3 minutes or until well browned but not burnt. Watch carefully! Let the shepherd's pie sit for 10 minutes before eating so that it can settle. Enjoy!