On a work surface, top 1 slice of bread with half the Kerrygold Dubliner® Cheese, the prosciutto, arugula and the tomato. Season tomato with salt and pepper. Top with the remaining cheese. Cover with second slice of bread and press sandwich together.
Brush top of sandwich with 1 teaspoon of the butter. Place buttered side down in a small nonstick skillet over medium-low heat. Spread top with remaining teaspoon butter. Cook 2 to 3 minutes or until the underside is golden brown. With a wide spatula, turn the sandwich over. Place a small heatproof plate on top and cook until the underside is golden brown and the cheese has melted, 2 to 3 minutes more. Alternatively, use the spatula to flatten the sandwich. Serve immediately.
*Other bread options: rye, semolina, white country