Pour about three-quarters of the warm milk-and-butter mixture over the potatoes. Mash with a potato masher until smooth and creamy, adding more of the milk mixture as needed to reach a soft, spreadable consistency. Taste and, if you like, add a pinch of salt, but remember the butter will add flavor too.
Stir the finely shredded cabbage directly into the hot mashed potatoes. The heat from the potatoes will start to soften the cabbage right away. Mix just until the cabbage is evenly swirled through, leaving some visible streaks of green for that classic colcannon look.
Spoon the potato and cabbage mixture into the prepared casserole dish, smoothing the top with the back of a spoon. Use the spoon to create gentle swirls and dips across the surface so there are little pockets for the butter to pool.
Cut the remaining 4 tablespoons of butter into small pieces and dot them all over the top of the casserole, tucking a few into the dips you made so they melt into golden pools.
Bake in the preheated oven for 20–25 minutes, or until the edges are lightly golden, the top is just starting to brown in spots, and the butter has melted into shiny puddles across the surface.
