Place the corned beef in a large pan and cover with water. Bring to the boil, then reduce the heat and simmer for about 3 hours or until tender, occasionally skimming any scum that rises to the top. Leave to cool in the pan and then about half an hour before you are ready to serve, reheat gently.
Preheat the oven to 200C (400F), Gas mark 6. Heat a roasting tin with the oil and butter. Add the carrots, tossing until evenly coated and season to taste. Roast for 15-20 minutes, turning occasionally until golden brown. Drizzle over the honey and parsley and pop back in the oven for another 2 minutes until lightly caramelised.
Meanwhile, make the colcannon. Cook the potatoes in a covered pan of boiling salted water for 15-20 minutes until tender. Five minutes before the end of cooking, add the cabbage. Warm the milk in a separate small pan with the spring onions. Drain and mash the potatoes and cabbage, then beat in the butter with the warm milk and spring onion mixture. Season to taste.
When ready to serve, remove the string from the hot corned beef and slice against the grain. Place good dollops of the colcannon on warmed plates and arrange the corned beef alongside with the honey roasted carrots next to it. Have a small dish of English mustard on hand to pass around.