Fill a large pot half way with water, and add the potatoes. Sprinkle some salt in the water. Once boiling, boil for 15-20 mins or until potatoes are fork tender. Depends on how big you cut the potatoes.
In a large, deep-sided skillet, pour the Guinness and turn heat to med. Add the bangers. Poach the bangers in Guinness.
Heat the oil and butter in a skillet and sauté the onion till nicely browned.
Remove the bangers and add to the onion pan for some browning.
Strain the potatoes and dump into a mixing bowl. Add the milk, cream, butter, salt & pepper and mix until mashed.
Serve the bangers & onions over the mash. Don't forget the Guinness to wash it down!