Place the Guinness, water and brown sugar into the pot and stir. Add the onion, followed by the corned beef and the spice packet. Cook the corned beef in the Instant Pot or Crock Pot according to the instructions for this Corned Beef and Cabbage recipe.

When the corned beef has finished cooking, allow to rest for 10 minutes. When cool enough to handle, slice against the grain.

Spread some whole grain mustard or horseradish on the top slider buns. Place the corned beef on the slider buns, top with coleslaw, and place the top half of the bun over the coleslaw. Secure with toothpicks, serve and enjoy!