1. Heat a cast iron skillet on medium-high heat until hot and steamy. While heating, spread Kerrygold Salted Butter on one side of the bread and cut in half to make enough for 1 sandwich. Toast on buttered side until golden. Remove from skillet and set aside.
2. Add in the cottage bacon (or leftover meat) and heat through, flipping to cook both sides until golden and crispy (about 5 minutes). On other half of the skillet, crack egg to fry. Use a spatula to contain the edges if needed. Carefully flip egg to cook other side slightly to keep runny consistency.
3. Place one slice of the bread on the serving plate, add the cooked bacon. Layer with cheese. Place egg on top, then a handful of arugula. Spread Dijon mustard on top slice of bread. Serve immediately.