Preheat oven and prepare pan: Preheat your oven to 425 degrees Fahrenheit and line a large sheet pan with parchment paper or lightly grease it to prevent sticking and promote even browning.

Season the potatoes: Place the diced russet potatoes in a large bowl, drizzle with olive oil, then sprinkle with salt and black pepper. Toss thoroughly until every piece is evenly coated for maximum crispiness.

Roast until crispy: Spread the potatoes in a single layer on the sheet pan, making sure they are not touching or overcrowded. Roast for 30 minutes, flipping halfway through, until the edges are deep golden brown and crisp while the centers stay tender.

Cook the bacon: While the potatoes roast, cook the bacon in a skillet over medium heat until crisp. Transfer to a paper towel-lined plate, cool slightly, then crumble into bite-sized pieces.

Warm the corned beef: Roughly chop or shred the corned beef and warm it gently in a small skillet for 2 to 3 minutes so it heats through and stays tender.

Make the roux for the cheese sauce: In a medium saucepan over medium heat, melt the butter completely. Whisk in the flour and cook for 1 minute, stirring constantly, until the mixture turns slightly golden and smells nutty.

Add milk and Guinness: Slowly pour in the milk while whisking continuously to prevent lumps, then add the Guinness stout. Simmer for 3 to 5 minutes until the sauce thickens enough to coat the back of a spoon.

Melt the cheese: Reduce the heat to low and gradually stir in the shredded cheddar cheese, one handful at a time, whisking until fully melted and smooth. Keep warm on low heat.

Assemble the nachos: Transfer the hot roasted potatoes to an oven-safe serving tray or skillet. Evenly scatter the warmed corned beef and crumbled bacon over the top.

Add the cheese sauce: Pour the warm Guinness cheese sauce generously over the potatoes and toppings, making sure it seeps into the layers for maximum flavor in every bite.

Bake briefly to melt: Return the tray to the oven for 3 to 5 minutes, just until everything is hot and bubbly.

Finish and serve: Remove from the oven and immediately top with sliced green onions and dollops of sour cream. Serve hot while the cheese is melty and the potatoes are crisp.