You can cook the corned beef in the Instant Pot, Crock Pot or a Dutch oven. For all methods, pour the Guinness, water and brown sugar into the pot and stir. Add the onion, corned beef and spice packet. For the Instant Pot, cook for 45 minutes on high pressure. For the Crock Pot, cook for 6 hours on low or 3 hours on high. For the oven, cook at 325°F(165°C) for 2 hours.
When the corned beef is done, pull it out of the pot and let it rest for 10 minutes.
Meanwhile, mix the coleslaw blend with the mayonnaise and rice wine vinegar.
Slice the corned beef against the grain.
To assemble the corned beef sliders, spread the mustard on the top buns. Place the corned beef on the slider buns, top with coleslaw, and add the top bun.
Notes
If your corned beef is thicker than 3 inches you will need to add cooking time to 60 minutes in the Instant Pot.