Put a pot of unsalted water on to boil for the potatoes.
Peel and chop the onion; set aside.
Put a frying pan on with the oil in it.
Wash potatoes; don't peel; chop into half-inch (1 cm) cubes. When the water has reached a boil, pop the potato in, lower the heat to a simmer, and simmer for 5 minutes.
When the potatoes go on to cook, pop the onion in the frying pan and stir to coat with the oil. You'll let the onion cook for 3 to 5 minutes over medium heat, until softened but not browned.
At this point, the potato should have just about had its time. Drain the potatoes, add the butter to the frying pan with the onion, then add the potatoes as well and stir in. Fry the potatoes for about 10 minutes or until their surfaces are crispy crispy and light brown.
Now roughly crumble the corned beef into the potato, along with the dried parsley, stir, and cook for another 5 minutes or so to heat everything through and crisp up some pieces of the beef.
Serve hot. Leftovers are great re-heated.