Preheat oven to 300 degrees.
Place corned beef, fat side up, and any liquid in its package in a dutch oven or large oven proof casserole with a top. If a separate spice pack was included, open it and pour it into the pan.
Add the onion, celery, bay leaves, 1 sprig of rosemary, garlic, peppercorns and whiskey.

Pour hot water (from the tap is fine) to just below the top surface of the corned beef so just a bit is exposed.
Cover the Dutch oven tightly with aluminum and place the top over the aluminum.

Place in the oven for 3 1/2 hours.
After corned beef has been in the oven 3 hours, toss the potatoes in olive oil and place them in a separate baking dish. Pull the leaves off the remaining sprig of rosemary and sprinkle on top.

Place dish in the oven along side corned beef and roast the potatoes for an hour (which will be an additional 30 minutes after you remove the corned beef) stirring occasionally.
After corned beef has been in the oven for 3 1/2 hours, remove the Dutch oven and transfer the corned beef to a work surface or cutting board.

Tent it with aluminum and allow it to rest.
Strain the liquid (or skim off the vegetables) and put it back in the Dutch oven.

Place it on the stovetop over a medium heat.
Add the carrots and cabbage, cover and simmer for 20-25 minutes or until vegetables tender.
Transfer vegetables to a serving platter.

Remove the potatoes from the oven and carefully drop them in the liquid for about a minute and add to the serving platter. Scrape any surface fat off of the corned beef and slice.
Add the slices to the platter.

Ladle a bit of the liquid over the meat and vegetables.
Sprinkle the parsley on top.
Serve everything with a side of horseradish and mustard.