Melt the butter with the garlic and milk. Set aside. Boil the potatoes (with skins, if desired) until fork tender. Drain the potatoe water. Mix the boiled potatoes with the butter/garlic/milk mix; add milk if needed to make standard mashed potatoes.
Heat gravy on low and add Guinness; do not boil.
Saute meat with onion and garlic for 10 minutes. Add salt and pepper as desired. Separate meat and grease.
Place meat in casserole dish.
Drizzle with 1/3 gravy/Guinness mix.
Layer with the can of carrot & peas.
Top with the garlic mashed potatoes.
Spread sliced tomatoes over the top and dizzle 1/3 of the gravy/Guinness mix over all.