Rinse corned beef well with cold water and pat dry with paper towels. Trim off excess fat. Place in pressure cooker insert. Pour in 1 cup (240ml) water. Pressure cook on high for 1 hour.
Remove cooked corned beef from pressure cooker insert. Shred corned beef with 2 forks. Set aside.
Heat 2 tablespoon vegetable oil in a cast iron skillet. Add potatoes and allow it to brown, turning occasionally. This should take about 8 to10 minutes. Then, add onions and continue to cook for another 3 to 5 minutes.
Add ½ teaspoon salt, ground pepper, and shredded corned beef. Stir to get everything well mixed, about 3 to 5 minutes.
Heat vegetable oil in another pan on the stove. Add mustard seeds and allow them to pop. Then add minced garlic for 20 seconds followed by cabbage wedges. Allow them to sear for 1 to 2 minutes before flipping over to sear the other side.
Add ½ cup (120ml) water and some ½ teaspoon salt (or to taste). Cover and allow cabbage to cook for 3 minutes. Turn off stove.
Transfer cabbage to skillet or a separate serving platter. Served with marbled rye bread and Irish mustard or horseradish.