This is an abomination, mixing in Guinness into a cottage pie.
The Guinness stew recipe parts are pulled from this recipe; the Shepherd's (lamb) and Cottage pie ingredients have been pulled from here and Epicurious. Cooking for Engineers has a nice pictoral guide.

For meat and vegetable filling

* 5 oz pearl onions
* 2 medium leeks (white and pale green parts only), cut into 1/2-inch-thick slices
* 1 lb meat - ground or cut into 1-inch cubes, ground beef or lamb shoulder
* 1 teaspoons salt
* 1/2 teaspoon black pepper
* 2 tablespoons all-purpose flour
* 2 1/2 tablespoons unsalted butter, softened
* 1 tbsp minced garlic
* 1 can Guinness
* 1/2 tablespoons tomato paste
* 1/2 cup beef broth
* 1/2 cup water
* a pinch of cayenne
* 3 bay leaves
* 1 teaspoons chopped fresh thyme
* 1 cup carrots, cut into chunks
* 1 cup peas
* 3 tablespoons flour

For mashed potato topping

* 1 lb russet (baking) potatoes
* 1/2 cup milk or cream
* 2 tablespoons unsalted butter
* 1/2 teaspoon salt

Preheat oven to 350°F.

Blanch onions in boiling salted water for 1 minute, then transfer to cold water to stop cooking. Peel.

If using chunked meat, pat dry, springle with salt and pepper then shake with flour to coat.

Melt 2 tablespoons butter in a sautee pan, brown meat. Add garlic and cook, stirring, 2 minutes. Remove meat and deglaze with 1/2 cup guinness. Add in tomato paste and boil until reduced by half. Add broth, water, meat (with juices), onions, leeks, carrots, peas, seasonings, and remaining guinness. Bring to a simmer over moderately high heat, add flour and reduce until thickened, then remove from heat.

If using chunked lamb, cover dish with foil and braise lamb and vegetables in middle of oven, stirring once or twice, until lamb is tender, 1 1/2 to 2 hours. Otherwise, continue to simmer over a low heat until the flavors have mingled.

For the potato topping:

Peel and quarter potatoes. Cover potatoes with salted cold water by 1 inch in a 4-quart heavy pot, then simmer, uncovered, until very tender, 20 to 25 minutes. Drain.

Add in milk/cream and butter to a simmer in same pot over moderate heat, stirring occasionally until butter is melted, then remove from heat and stir in salt and pepper. Mash finely using a ricer.

Optional, from Epicurious: Make a beurre manié by stirring together remaining 1 1/2 tablespoons butter and remaining 2 tablespoons flour in a small bowl to form a paste. Spoon 1 cup cooking liquid from casserole dish into a small saucepan and bring to a boil. Whisk in beurre manié, then simmer, whisking occasionally, until thickened, about 2 minutes. Gently stir sauce into lamb and vegetables.

Spoon potatoes over lamb and vegetables and spread evenly with a fork, making a pattern with tines. Broil about 3 inches from heat until top is golden, about 3 minutes.