If you think you have time to go to the store and pickup some corned beef, cabbage, rye bread and/or potatoes, and get it done in time for dinner, it ain't gonna happen... but... if you're looking at a 9pm SUPPER, you have time!
I searched the web to death yesterday and I found THE BEST method of cooking your Corned Beef and Cabbage - the link is here. If you are pressed for time, best thing to do is get that piece of beef and get home in hurry... drop it in a big pot of boiling water, add bay leaf and pepper (or that seasoning packet if your brisket comes with it)... KEEP BOILING ON HIGH FOR ONE HOUR (I know the package says turn it down after 10 minutes, but you wanna eat before 9, right?)
Then, down to medium boil. Cook for 3 hours. Check and add water every now an dthen if you notice it is getting too low. About 15 minutes before it's done, clean and cut up your cabbage and toss in the pot.
You can boil the potatoes in a pot of their own (small reds work best) or nuke 'em in the microwave on HIGH, put 'em in about 12 minute sbefore dinnertime! Don't forget the brewski and you'll be good to go!