In a large pan, set over medium heat, melt 4 tablespoons of Kerrygold unsalted Irish butter, and sprinkle the flour over the melted butter. Then, quickly whisk the melted butter and flour together for about a minute to form a light roux.
Next, add the Guinness Extra Stout beer and quickly whisk until it's thick and bubbly.
Then, add the whole milk and half & half, whisking well after each addition.
Next, whisk in the Dijon mustard, salt, and pepper. Continue to cook, bringing the sauce to a simmer for a couple of minutes, whisking frequently.
Then, cut the room temperature cream cheese into small pieces and add to the sauce, whisking constantly until it's all completely melted and incorporated.
Next, add 5 ounces of shredded Kerrygold Reserve Cheddar cheese and whisk constantly until it has all melted and the sauce is smooth and creamy.
Then, whisk in 5 ounces of Kerrygold Dubliner shredded cheese. Continue whisking until all the cheese has melted and the sauce is creamy and smooth.
Turn the heat down to low.
While you're making the cheese sauce, cook the pasta according to package directions, minus one minute from al dente, reserving one cup of pasta water. Drain the pasta. Then add the pasta to the sauce and stir until the pasta is completely coated in the cheese sauce.
Add the remaining 2 ounces of Kerrygold Reserve Cheddar and 2 ounces of Kerrygold Dubliner shredded cheese. Stir everything together until all of the cheese has melted.
Next, add the reserved pasta water and stir until all of the water has been incorporated. Let the mixture sit over low heat, stirring occasionally to allow the pasta to soak in some of the cheese sauce while you make the topping.