Add potatoes to Instant Pot with cooking stock, cover with lid, ensuring that the valve is set to sealed, press manual and set the cook time to 15-minutes.
While the potatoes cook. Warm butter, milk, cream and a few cracks of black pepper over low heat in a small saucepan.
After the timer goes off allow natural release for 5 minutes and then vent. Remove potatoes from the instant pot with tongs and set aside until cool enough to handle.
Add thinly sliced cabbage into the Instant Pot, replace the lid, ensuring that the valve is set to sealed, and set to manual high pressure 6 minutes.
As the cabbage cooks, slip the skins off of the potatoes and cube. Set aside.
Once the cabbage is done, press Cancel on the IP, safely do a Quick Release by opening the valve to "Vent". After all of the pressure has been released open the lid. Scoop out the cabbage and set aside.
Pour the excess stock into a container and reserve for corned beef. Add the potatoes, cabbage, sliced green onions, and half of the cream mixture back into the Instant Pot. Using a hand masher, mash the Colcannon adding more cream until the desired consistency is reached, it should be "lumpy".
Serve warm with a knob of butter on top if desired and seasoned with Kosher sea salt and freshly cracked black pepper.