Thanks for the recipe - I make Shepherd's Pie every year for St. Patrick's Day (i am not a fan of corned beef - it's not traditional in Ireland anyway!) I made a few modifications to your recipe (because I found an awesome recipe a few years ago and figured I could try to mash the two)
I reduced the oil to 1 tbsp and added an a tbsp. of butter (Kerrygold, of course!), I also took out the cup of tomatoes and replaced it with 2 tbsp of tomato paste (adds a nice richness to the beef). I increased the onion to one whole yellow onion and added a tsp. of black pepper. Other additions are 1 tbsp of Worcestershire sauce, 1/2 cup of chicken stock, 1 tbsp of Xanthum gum and a glass (3.5 oz) of red wine.
The rest (cauliflower mash) remains the same as your recipe.

Total calories: 398; 8g of carbs, 28g of fat and 22g of protein