Preheat oven to 425°F. Line two large baking sheets with parchment paper. High heat is what separates crispy Irish nachos from soggy potato rounds, don't drop below 425°F.
Slice potatoes into ¼-inch rounds using a mandoline or sharp knife. Dry them thoroughly with paper towels, surface moisture is the enemy of crispiness.
Toss potato slices with 1 tbsp olive oil, salt, and black pepper in a large bowl until every slice is lightly coated.
Spread in a single layer across both baking sheets, no overlapping. Crowded potatoes steam each other and turn soft. Single layer only, every time.
Roast at 425°F for 20–22 minutes, flipping once at the halfway mark, until the edges are golden and the centers feel firm when pressed.
Pull the sheets from the oven. Arrange the crispy potato rounds on one baking sheet, slightly overlapping like nachos, edges touching is fine at this stage.
Scatter shredded cheddar generously over the entire layer. Every potato round should have cheese coverage, bare spots go cold and dry while everything else melts.
Sprinkle chopped bacon evenly over the cheese layer.
Return to the oven for 4–5 minutes just until the cheese melts and bubbles. Watch it, you want melted and golden, not browned and rubbery.
Remove and immediately top with dollops of sour cream and any optional garnishes. Serve straight from the baking sheet, Irish nachos wait for no one.