Combine the diced potatoes, 1 tablespoon oil and ½ teaspoon salt and ½ teaspoon pepper in a microwave safe bowl. Cover the bowl and microwave on high, stirring occasionally, until the potatoes begin to soften, 5 to 7 minutes. Drain.
Warm a large cast iron skillet over medium heat for a few minutes. Add 1 tablespoon oil and heat until shimmering.
Add the onion and cook until softened and starting to brown. Add the garlic and sauté until fragrant, about 1 minute. Transfer to a bowl and keep warm.
Pour the remaining 1 tablespoon of oil into the skillet. Once shimmering, carefully add the potatoes and spread out to cover the pan. Cook, without stirring, until the potatoes are browned, about 4 to 5 minutes.
Flip about ¼ of the potatoes at a time with a large spatula and gently press into the skillet. Repeat with the remaining potatoes. Cook until the potatoes are crisp on the outside and tender on the inside, about 10 minutes.
Return the onion mixture to the skillet along with the corned beef. Cook until heated through. Add the hot sauce and 1 or 2 tablespoons water to the pan. Toss to combine. Remove from the heat and add fresh parsley, if using. Season with salt and pepper and serve.
Serve with eggs and buttered toast.