Place the potatoes in a large saucepan of water. Bring to the boil, then lower the heat and simmer for 15-20 minutes, or until the potatoes are tender when pierced with a fork.
While the potatoes are cooking, melt a quarter of the butter in another saucepan over medium heat. Add the sliced onions, crushed garlic, and half of the shredded cabbage. Sauté for about 5 minutes, or until the vegetables are softened. Remove from heat and set aside.
When the potatoes are done, drain them in a colander.
Mash the potatoes until they are smooth. In a small saucepan, heat the double cream with the remaining butter until almost boiling. Then beat this mixture into the mashed potatoes.
Stir the sautéed onions, garlic, and cabbage into the mashed potatoes. Mix well to combine.
Season the colcannon with nutmeg, salt and pepper to taste. Serve warm, with gravy if you like.