Mix together the corned beef, cheese, salad dressing, sweet pickle relish, and onion.
Spread the onion rolls or hamburger buns with margarine or butter.
Spoon the sandwich filling into the rolls.
Wrap the sandwiches in heavy foil.
Grill over medium-hot coals for 12 to 15 minutes, turning sandwiches several times, or bake at 375° till hot through, about 20 minutes.