Preheat oven to 250 degrees.
Place ribs on two separate pieces of heavy aluminum foil, shiny side out.
Sprinkle fajita seasoning and brown sugar evenly over ribs, rubbing in.
Cover each rack of ribs with another piece of foil, shiny side out, folding foil on 3 sides so that it seals in the ribs.
Pour 1/2 cup Guinness sauce evenly over each rack.
Seal the other side of the foil.
Bake in 250-degree oven for 3-3 1/2 hours.
Once tender, remove the top piece of foil.
Pour off juices from the packet.
Bring to boil and let simmer until reduced by half.
Brush on reduced juices and broil until ribs are slightly crispy.