Some people cook their vegetables alongside their corned beef, however steaming them apart gives them a cleaner flavor. This is important, because the Irish have a library full of sauces that taste great over vegetables.
For the vegetables:
Parsley sauce
Course mustard sauce
Guiness mustard sauce
Onion sauce
For the corned beef:
Dill pickle-horseradish sauce
Béarnaise sauce, though French, works for corned beef
Cream horseradish sauce
I try to get the best of both worlds: I steam the cabbage with water from the Brisket.