Let me start by pointing out that corned beef and cabbage is not a traditional Irish dish, it is a traditional Irish-American dish. In Ireland, you would be much more likely to see boiled bacon and cabbage. Of course, this isn't the bacon you're probably thinking of, but that's beside the point. Whether the Irish really eat Corned Beef or not on St. Patrick Day, it has become the meal of choice around the world.
The secret to grilling a corned beef is to make sure you've soaked it well to lift out the salt before you put it on the grill.
Corned beef is a beef brisket that has been salted and seasoned to preserve it. In fact, the term “corned” comes from the very coarse salt used long ago in this process. In the Old English, corn referred to grain (American corn or maize was still unknown to Europe) about the size of the coarse salt used, hence, “corned” beef. The secret to cooking corned beef is to get that salt back out. This is typically done by boiling it with potatoes and other vegetables. Potatoes are particularly good at absorbing salt. Tradition dictates that cabbage is then boiled in the water you boiled the beef it. This flavors the cabbage.
Soak: To start off grilling our corned beef you need to soak the beef for about 30 minutes per pound and no less than 2 hours. This soaking needs to be done with warm water or the salt will not dissolve effectively.
You also need to change the water every hour and rinse the corned beef off when you change the water. You may choose to place the corned beef in a pot of water and set it on your stove over a low heat for the soaking time.
Season: Once the corned beef has been soaked and rinsed off you can now apply the seasonings.
Corned beef is frequently sold with a spice packet. You can use this as a rub on the meat. You can also mix up your own rub. Use coarsely cracked black pepper, cracked coriander seeds, onion powder, thyme, paprika, garlic powder, and cayenne. Typically you want a lot of pepper and coriander and less of the other ingredients. Try 4 parts pepper and coriander and 1 part of the others. You can, of course, add whatever you want, but corned beef is traditionally seasoned with peppercorns and coriander so if you want the authentic thing flavor, use these. Rub the spices into the meat. You want it to get deep inside so the flavors will spread.
Grill: Now you are ready to grill. You want to grill your corned beef for about 1 to 2 hours at a low temperature (around 250 degrees F. (120 degrees C.)). Charcoal and hardwoods will give you better flavors but you can use a gas grill. Either way, go for indirect grilling and use a drip pan under the corned beef. Grill until the internal temperature of the meat reached more than 165 degrees F. (75 degrees C.). If you simmered the corned beef in hot water first it is probably already around this area. What you want to get is a corned beef that has lightly crusted on the surface but hasn't dried out so watch the meat closely, especially after an hour.
Baste: During the grilling, you want to baste the corned beef every 30 minutes. Use some of the rub that you prepared before with equal parts of water, oil, and vinegar to makes something similar to a salad dressing. This will keep the meat moist and also help drain out any additional salt left behind. By basting with seasonings from the rub you continue to add that great flavor to the corned beef. If you introduce some smoke to the process you'll add even more flavor.
So, why would you want to grill a corned beef? Well, you add a lot more flavor to the meat than you would simply boil it. Also, you get a piece of meat that isn't boiled to a mush. Grilled corned beef is denser and has a much better texture to it. If you want to get the cabbage to go with this use the last batch of water that you soaked the corned beef it.
Bring it to a boil, throw in some potatoes and then the cabbage. You can also add some of the drippings from the drip pan in your grill. This will really flavor up the mixture.