Peel the onions and cut across making thick rings. Separate and place the rings into a large bowl of ice water for about 15 minutes.
While the onions soak, in a small bowl whisk together all of the ingredients for the dipping sauce. Cover and refrigerate until ready to serve.
Heat your oil to 350˚F. Place cornstarch in a large bowl. In a second bowl mix together flour, paprika, baking powder, garlic powder, salt and pepper to paste. Whisk in enough Guinness to form a lump free batter close to pancake batter.
Remove the onions from the water and pat dry. dredge in the cornstarch, then the Guinness batter. Place in heated oil and allow to fry for about 1 to 2 minutes on each side until golden brown. Remove from oil and place on a paper towel.