from someone who's on it everyday, the red cow roundabout is fine. as long as you dont hit it inbound between 7-9:30am and 5-6pm, its really your fastest way to dundrum. As above, as you come up to newlands stay in the right hand lane all the way down to the red cow.
If you really want to take the belgard road, I wouldnt go all the way to the the tallaght bypass, once on the belgard road, turn left at what should be the second set of lights to bring you down to the M50 on the cookstown road.