(Irish pork belly has a fatty layer, vital for the flavor, and a meaty layer. See if you can get your butcher to give you pork that has about equal amounts of pork and fat. Plain lean pork will not taste as good.) Cut meat into one-inch cubes. Peel and thinly slice potatoes. Peel or wipe mushrooms, and slice if large. Grease a large ovenproof dish with some of the butter, and place a layer of sliced potatoes in the bottom. Add half the meat pieces, seasoned and sprinkled with rosemary. Layer with half the vegetables, plus another layer of potatoes. Repeat, using all meat, veg and potatoes. Pour stock over and dot potatoes with remaining butter. Cover and cook in a moderate oven, 325F, for 2 1/4 hours. Uncover for the last half hour of cooking to brown the potatoes.