Preheat oven to 350° F.
For the filling, place a large skillet over medium-high heat with EVOO, 2 turns of the pan. Add onion and sauté until soft and slightly caramelized, 6-8 minutes. Add garlic and corned beef to the skillet and season with salt and pepper.
Meanwhile, potatoes into a large pot of salted water and bring up to a boil. Let potatoes boil for about 5 minutes, until tender. Drain and let cool.
Once the potatoes are cool enough to handle, cut the top third off and scoop out the guts of the potatoes, using a small spoon or melon baller. Be careful not to puncture through the sides or bottom of the potato. Place the guts into a large mixing bowl and set aside. Discard the extra potato tops.