Prepare macaroni according to the package. Grease a 9×13 dish and preheat the oven to 350F.
In a large frying pan on medium heat, sauté onion, garlic, pimento and herbs for 2 to 3 minutes. Then add corned beef and sauté for a further 5 minutes before setting aside.
In a large bowl add water and Dairy Dairy, whisk thoroughly to combine.
To the Dairy Dairy, add the eggs, onion powder, garlic powder, salt, black pepper, mustard, parsley and butter, whisk to combine before adding 3 cups of cheese.
Pour the Dairy Dairy mixture over the cooked macaroni and gently fold it in until fully coated.
Add half of the macaroni to the baking dish then top with all of the corned beef, evenly spreading it out. Then add the remaining macaroni and top with 1 cup of cheese.
Bake macaroni pie for 30 minutes and then broil for 2 to 3 minutes, until the top is golden.
Remove from oven and allow to cool for at least 30 minutes before serving.