Place potato pieces into an empty pot. Fill with cold water until there is an inch or two water above the potatoes. Season the water with a little salt and then bring to a boil. Boil the potatoes for about 7-10 minutes until the potato edges are soft (almost fork-tender).
When the potatoes are almost fork-tender, drain them and add them to a large mixing bowl. Gently smush some of the potatoes with a fork. Also to the bowl, add the corned beef, sliced onion, garlic powder, paprika, parsley, salt, and pepper. Toss gently to coat everything in the seasoning.
In a large cast iron skillet (or non-stick skillet), melt the butter over high heat. Add the corned beef mixture, spreading it out into a single layer. With a spatula, gently press down the hash. Let the hash cooked, undisturbed, for a few minutes before flipping it.
Continue pressing down the hash, letting it cook, and then flipping until the hash is crispy and golden, about 15 minutes. Taste for seasoning, adding more salt or pepper if necessary. Optional: top with fresh chopped parsley or fried eggs, if desired. Enjoy!