Place the potatoes on the sheet pan and bake for an hour or until a fork easily passes into the middle of the potato easily.
While the potatoes are cooking heat, on medium, 2 tablespoons of butter and add the onions. Sauté for 10 minutes or until the onions become translucent. Add the Guinness and cook until it has reduced until most of the liquid has evaporated. There will be some liquid remaining and that is a good thing. Set aside.
Once the potatoes are done remove from the oven and reduce the heat to 350°F or gas mark 4.
Using a sharp knife cut the potatoes in half. Use a spoon to carve out the inside of the potatoes leaving at least a half a centimeter around the edges. Put the potato that was carved out into the mixing bowl. Once all of the potatoes have been carved out and the removed potato is in the mixing bowl mash the potatoes and then add the remaining butter, the sour cream, Dubliner cheese, sour cream and onion mixture. Mix until everything is combined evenly.
Using a piping bag fill the potatoes with the mashed potato mixture. Bake for another 30 minutes.