Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Add 1 tablespoon butter and olive oil and allow butter to melt. Add cabbage and green onions and cook until wilted, about 2 minutes. Turn pot off.

Add potatoes and water to the pot. Close and lock the lid. Select manual pressure and set timer for 5 minutes. Allow about 10 minutes for pressure to build.

Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.

Remove pressure cooker liner and place on the counter. Add milk and mash using a potato masher. Let colcannon stand for 3 minutes.

Transfer to a serving dish. Make a well in the center of the potatoes and add remaining 2 tablespoons butter. Season with salt and pepper to taste.