Start by melting unsalted butter in a bowl, then mix in Irish whiskey to create a flavorful marinade. Coat the sirloin steaks thoroughly with this mixture and let them marinate in the refrigerator for at least 3 hours or overnight.

In a cast iron skillet, heat a tablespoon of oil over medium-high heat. Carefully place the marinated steaks in the skillet, searing each side for 3-4 minutes until a deep brown crust forms.

Preheat your oven to 400°F. Once the steaks are browned, transfer the skillet to the oven. Cook for an additional 4-6 minutes for medium-rare.

While the steaks are in the oven, bring salted water to a boil and add peeled, chopped potatoes. Cook until tender, about 15-20 minutes. Mash the potatoes and mix in sautéed kale.

In the same skillet used for the steaks, sauté chopped shallots over medium heat until translucent. Pour in whiskey and let it simmer. Stir in full-fat cream and reduce until thickened.

To serve, plate the colcannon and nestle the sliced steak beside it. Drizzle the whiskey cream sauce over the steak and garnish as desired.