Preheat oven to 375°F.
Bring a large pot of water to boil and add 1 ½ teaspoon Kosher salt. Add the elbow macaroni and cook according to package directions for al dente minus two minutes. Drain in a colander and set aside until ready to use.
After you've started the pasta water, begin making the cheese sauce. Start by melting butter in a large saucepan over medium heat. Once the butter has melted and is gently bubbling, add flour, mustard powder, freshly ground black pepper, and the remaining ½ teaspoon of Kosher salt. Gently whisk to incorporate and smooth out any lumps. Continue whisking frequently until the roux turns a light blond color and emits a nutty aroma.
Reduce heat to low. Then, slowly stream in room temperature milk and continue whisking until the mixture is smooth. Next, add the Guinness and continue whisking until the sauce is smooth and thickened, for about 5-7 minutes.
Add five ounces each of shredded Kerrygold Dubliner and Aged Cheddar cheeses, a handful at a time, whisking well after each addition, until all of the cheese has been added and the sauce is smooth.
Once the sauce is finished, place the cooked pasta, three ounces of pastrami, and six slices of cooked and crumbled bacon in a large bowl, stir everything to combine. Pour the cheese sauce into the bowl and stir to combine.
Spray a 2-quart baking dish with non-stick cooking spray. Transfer the pasta mixture to the dish and spread it into an even layer. Top with remaining ounce of pastrami, four slices of crumbled bacon, two ounces each of the Kerrygold shredded cheeses.
Bake in 375°F oven for 25-30 minutes, until cheese bubbles and is slightly golden. Remove from the oven and allow to cool for 5-10 minutes before serving.
Yield 6 servings