Melt 1 1/2 tablespoons butter in a large pan over medium heat. Add onion and sprinkle with sugar, 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook until soft and golden, stirring occasionally, about 10 to 15 minutes.
Add beer and continue cooking until liquid turns slightly syrupy, about 8 to 10 more minutes. Remove from heat and stir in Worcestershire sauce and mustard. Mix in the corned beef slices.
On 4 slices of bread, layer the cheese, and corned beef and onion mixture. Top with the remaining 4 slices of bread.
On a griddle or large pan, melt some butter and grill the sandwiches until lightly browned and crispy on both sides and cheese is melted.