Fill a Dutch oven, or large pot, with 8 to 10 cups water and and 1 teaspoon salt. Set pot to boil on medium/high heat. Once water is boiling reduce heat to medium and add the potatoes. Boil until almost tender (10 to 12 minutes). Drain potatoes and set aside.
While potatoes are cooking, chop the cabbage and thinly slice the green onions.
In the same pot that was used for the potatoes, add 6 tablespoons butter and melt over medium heat. Add cabbage and green onions and cook until chopped cabbage has wilted and released its water (about 3 to 5 minutes).
Add milk, potatoes, salt, and 2 tablespoons butter to the cabbage and onion mixture. Using a potato masher, blend all ingredients together well incorporated and fluffy. Taste for seasoning. Serve Traditional Irish Colcannon warm with a pat of butter.
Notes
Recipe Variation: Don't like cabbage? That's okay. A lot of people use kale in their colcannon. If using kale, the recipe will remain generally the same. Wash, dry, and chop the kale. In a Dutch oven, melt the butter and cook the kale and onions until wilted (about 5 minutes). Add the rest of the ingredients and mash.