In small bowl, mix vinegar, oil and pepper. Lay onion and pepper rounds on a plate and brush generously with oil and vinegar mixture. Rub remaining mixture all over steaks. Let marinate 5 to 15 minutes.
Lay steaks on an oiled barbecue grill over a solid bed of hot coals or high heat on a gas grill. Set onion and pepper slices alongside. Close lid on gas grill. Cook steaks 5 minutes and turn over. Top steaks evenly with Kerrygold Dubliner® Cheese and continue cooking until done to your liking in center of thickest part (cut to test), about 5 to 7 minutes longer for medium-rare. Cook onion and pepper slices, turning once, until lightly browned on both sides, about 6 to 10 minutes total. Transfer steaks to plates. Separate onion into rings and place on steaks, along with pepper slices. Let rest in a warm place 3 to 5 minutes before serving. Add salt to taste.