• In a small bowl,combine mustard and brown sugar. Brush all over corned beef brisket.
• Place corned beef in steamer basket of the Deluxe Multi Cooker with Guinness Stout, set to CUSTOM, 90minutes. Press button to start. Once timer goes off, allow pressure to naturally release for 10 minutes.  Press steam button to release any additional pressure, and remove corned beef to cutting board.
• Allow corned beef to rest an additional 10minutes, then shred.

For the Potatoes

• Wash potatoes, then slice, unpeeled, into 1/2" thick rounds. Place slices into a large pot of cold water, ensuring that they are cover completely.  Bring to a boil over high heat. Boil only until just tender, about 5 minutes.  To test this, inserting a knife into the middle of a potato should meet with a bit of resistance.  Drain and dry on baking rack or with paper towels.
• Preheat oven to 425°F/220⁰C.
• Line two sheet pans with foil. Place potatoes in a single layer divided evenly between both pans.  Drizzle with oil and sprinkle with oregano, garlic, salt and pepper, coating both sides of potatoes.
• Bake for 30 minutes, or until golden and crisp.
• One on of the sheet pans, bring the potatoes together in a mound.  Top with half of both cheeses, red onion, beans, and shredded corned beef. Top with potatoes from the other tray, adding remaining cheese, red onions, beans, and corned beef. Place tray back in oven and bake an additional 5 minutes, until cheese is melted.
• Combine sour cream and horseradish in a small bowl.
• Remove nachos from oven, and top with dollops of sour cream, pickled jalapeños, and chives.