Potatoes and cabbage are the logical choice to serve up with corned beef. I am not fond of plain boiled potatoes or cabbage (or really any vegetable for that matter), but put together a pseudo colcannon recipe that worked out really well.
Boil potatoes. While they boil, cook 4 strips of bacon. Once cooked, removed from pan with a slotted spoon. In the bacon fat, cook up 1 onion and half a small cabbage (chopped). Once potatoes are cooked mash them with 1/4 cup of melted butter and some milk. Mix in the bacon and onion/cabbage mixture.
Voila, the perfect corned beef cabbage accompaniment.