Preheat oven to 350 degrees F. Spray a 3-quart baking dish with non-stick cooking spray.
Pour the 2 Irish Lagers into a large sauce pot and bring to a boil. Reduce heat and simmer until the beer has reduced by half. Remove from heat and set aside.
In a large pot, bring 4 quarts of water to a boil; add 1 tablespoon salt. Pour in the pasta and stir, cook for 6 minutes, stirring occasionally. Drain pasta in a colander and set aside.
Meanwhile, in a large sauce pot, melt 4 tablespoons of butter over medium heat. Stir in onions and saut until onions are soft, about 4 minutes. Add the garlic and continue to saut for 2 minutes more, stirring constantly. Whisk in flour and cook 3 minutes, whisking constantly. Add the reserved beer, half-and-half and bring just to a boil, whisking constantly. Reduce heat and simmer, whisking often until sauce slightly thickens, about 2 minutes. Reduce heat to low. Add 12 oz White Irish Cheddar cheese and 8 oz Medium Cheddar and stir until melted and smooth. Stir in the salt, pepper and cayenne. Remove pan from heat and gently stir in the rotini and tomatoes. Pour the pasta mixture into prepared baking dish.
Melt 2 tablespoons butter and toss with the panko until well coated. Evenly distribute the panko mixture over the pasta. Bake for 25-30 minutes, or until the cheese is bubbly and the topping golden brown.