In a 12-inch sauté pan, brown the bangers on all sides over medium heat.
Add Guinness to the pan and reduce heat to a simmer. Braise the bangers until fully cooked and the beer has reduced by half, about 10 minutes.
Remove the bangers and whisk in the brown sugar, butter-flour mixture, and salt until the sauce is smooth.
Bring to a gentle boil, then reduce to a simmer until thickened. If the gravy thickens too much, add 1 to 2 tablespoons of chicken stock.
Place potatoes in a medium saucepan, cover with cold water, and add 1 ½ tsp Kosher salt.
Bring to a boil over medium-high heat and cook until fork-tender, about 10-12 minutes. Drain and keep warm.
While the bangers are cooking, heat a 12-inch skillet over medium heat and cook the bacon until almost crisp, rendering as much fat as possible.
Add 2 tablespoons of butter and allow it to melt, then stir in the cabbage. Sauté until the cabbage is tender but still has some crunch, about 10-15 minutes.
Mash the potatoes with the remaining butter, heavy cream, and ½ tsp black pepper until smooth.
Fold in the cabbage and bacon mixture.
Plate the colcannon and top with the Guinness-braised bangers.
Drizzle with the rich brown sugar Guinness gravy and serve hot.
