Skin the potatoes and cut them into about 8 chunks so they cook faster. Drop the potatoes into a large pot then add cold water to just an inch over the potatoes. Add a half teaspoon of salt to the water. Turn the heat on to high, and bring the water to a boil. Reduce the heat to low to maintain a simmer and cover. Cook for 15 to 20 minutes, or until you can easily poke through them with a fork.
While the potatoes are simmering, brown the beef and lamb in a little olive oil, working in batches if necessary.
Add a bottle of Guinness to the meat and simmer until the liquid has almost all absorbed and evaporated.
Remove the meat and add it to a medium-sized bowl.
When the potatoes are pokable, drain the water and put them in a large bowl. Pour 2 Tbsp milk and 2 Tbsp butter over the potatoes. Mash the potatoes and use a strong wooden spoon to beat further. Add the rest of the milk and beat until the mashed potatoes are smooth. Don't over-beat the potatoes or the mashed potatoes will end up gluey. Add salt and pepper to taste and set the mashed potatoes aside.
With the pan on low heat, fry the onions, celery and carrots in olive oil until the onions become translucent. Add the garlic, cornstarch, and tomato paste, and stir until the tomato paste evenly coats the veggies. Cook for about 4 minutes.
Add the meat back to the pan and pour over remaining Guinness and let it reduce slightly before adding the beef stock and Worcestershire sauce. Place the herbs on top in bunches, making sure not to mix them in, or put them in a tea ball and submerge them. Just make sure you can fish out the herbs later. Bring the meat mixture to a simmer, and cook, uncovered, for 30 minutes, or until the liquid becomes gravy-like. If the sauce is not thickening, add some cornstarch mixing it in in small batches.
Preheat the oven to 400 degrees F.
Season the mixture generously with salt and pepper, and remove the bay leaf, rosemary, and thyme.
Add the peas to the mixture. Mix well. Pour into a casserole dish or ramekins if you want to make individual pies.
Generously top the meat mixture with mashed potatoes. Use the back of a spoon to smooth them over, and to make sure that they cover lip of their dish. Sprinkle the shredded Dubliner cheese on top of the potatoes.
Bake for 20 minutes, and let cool for about 5 minutes before serving.
