1. Preheat oven to 375 degrees.
  2. Heat oil in a large saute pan over medium heat. Add lamb and cook until no longer pink. Reserve 1 tablespoon rendered fat and drain meat. Add reserved fat back to the skillet and place over medium heat. Add carrots and 2 cups chopped onions, stirring occasionally, until vegetables start to soften, about 5 minutes. Stir in tomato paste and cook for a minute. Stir in flour and cook for another minute.
  3. Add lamb back to pan along with Worcestershire, beer and chicken broth. Bring to a low boil and cook until liquid is reduced to a thick gravy, about 15-20 minutes. Season to taste with salt and pepper. Remove from heat and stir in peas.  Spread meat mixture into 9 x 13 pan.
  4. Cook potatoes in boiling water about 20 minutes or until very tender; drain well and mash with skins on, adding milk and 3/4 teaspoon salt.
  5. While potatoes are cooking, melt 6 tablespoons butter in a large skillet. Add remaining 1 cup onion; cook 10 minutes, stirring occasionally, until very soft. Add cabbage; cook and stir for 5 minutes more or until very soft.
  6. Stir cabbage mixture and Dubliner cheese into hot potatoes and season with pepper. Spread Colcannon on top of lamb mixture in casserole dish.
  7. Melt remaining 2 tablespoons butter and brush evenly over Colcannon. Place casserole dish on a baking sheet and bake for 30-35 minutes or until edges bubble and potatoes start to brown. Allow to stand 10 minutes before eating.