In a large pot over medium heat, add 2 tbsp olive oil.
In small batches, brown the lamb and reserve in another container.
Add onions to the pot to lift the fond and cook until translucent.
Add the mushrooms and cook until soft,
Add the garlic and cook until fragrant.
Toss in the carrots and potatoes, and add in the tomato paste. Cook for 1-2 minutes.
Add the meat back in and combine.
Pour in one and a half cans of Guinness and bring to a simmer. Stir in rosemary.
Once the pot has come to a simmer, add 1L beef stock, season with salt and pepper again, and simmer on medium low uncovered for 1-2 hours.
Using 2-3 tbsp of the stew liquid in a separate bowl or ramekin, whisk in the cornstarch until it dissolves. Add it back into the stew and cook for 5-10 minutes, or until thickened.
Toss in the peas and cook until just heated through.
Stir in the parsley and serve with some Irish soda bread!