It might not seem to matter too much how you slice your meat, but it can actually make the difference between an enjoyable meal and one that is just plain hard to chew. Corned beef, which is a good source of protein, iron and zinc, is made from the muscle of a cow, and that muscle is formed with a variety of fibers that attach together. When you slice against the grain, you're shortening these fibers, which makes the meat more tender and easier to chew.
Place your cooked piece of corned beef on a large cutting board. Make a small incision along one end of the corned beef. Pull the meat apart, using a fork if necessary, to see which way the grain fibers of the meat run.
Arrange the piece of corned beef on the cutting board so the fibers of the meat are situated diagonally to your body.
Use a sharp knife to trim away the fat around the edges of the corned beef. You don't want to eat the fat because it's unhealthy, and too much fat contributes to an increased risk of heart disease. Removing the fat before slicing the meat is easier than trying to cut it off of each individual slice of corned beef.
Slice the corned beef by cutting the meat at right angles to the direction of the grain, according to the well-known cooking book "The Illustrated Cook's Book of Ingredients." Cut the meat into thin slices as another way to shorten the muscle fibers and to help make the meat more tender than you would get with thicker slices.