Place margarine in skillet, melt, tilt skillet to cover bottom evenly. Add potatoes & onion, stir to coat. Add salt & pepper. Place lid on skillet, turn heat to low and cook potatoes & onion, stirring occasionally to keep from sticking, until onion is "clear". Open corned beef, break apart, put into skillet, stir to blend. Add warm water, stir. Cover to steam until water is absorbed.