Preheat your oven to 400°. Place the bread onto a rimmed, metal sheet pan. Brush both sides with olive oil and sprinkle with a few pinches of kosher salt. Bake for 10 to 12 minutes. Flipping the slices halfway through. Once cool, cut the toasts in half.

Meanwhile, heat a 10-inch cast iron skillet on medium-low and melt 2 tablespoons of butter. Add in the sliced onions, sprinkle in a generous pinch of salt and toss to coat. Cook slowly, tossing occasionally until the onions are softened and eventually turn golden brown about 25 minutes.

In a medium bowl, add the cream cheese, 4 ounces of the grated Dubliner cheese, mayonnaise and the caramelized onions. Pour in the Guinness and stir until all of the ingredients are incorporated.

Spread the mixture back into the cast iron skillet, sprinkle with the remaining cheese and bake for 25 to 28 minutes. Allow the dip to cool for 10 to 15 minutes before serving with the toasted rye slices.