Set your Instant Pot to the sauté function. Add the olive oil and heat for a minute. Add the shredded cabbage and minced garlic. Sauté for about 3-4 minutes.
Peel and chop the Yukon gold potatoes into 1-inch cubes.
Once the cabbage has softened, add the chopped potatoes, water, salt, and pepper to the pot. Stir gently to combine.
Secure the lid and set the valve to sealing. Cook on high pressure for 10 minutes. After cooking, allow for a natural release for 5 minutes before venting any remaining pressure.
Open the lid and drain any excess water if necessary. Add heavy cream and unsalted butter to the potatoes. Mash until smooth and creamy.
Spoon the colcannon potatoes into a serving dish. Drizzle with extra melted butter and garnish with chopped chives or parsley if desired.