Look at it again, McGrath makes contact with his upper body with the Irish player lying on top first. He's decelerating before contact with the Baabaa's player which would reduce the force of impact hugely. Compare this to Pape, although he is considerably lighter than McGrath at 115kg and that the weight of the offender should definitely be taken into account by the citing commission it's still far worse.
Heaslip is in a maul using his legs to keep him upright so again there is no give, another vital factor to be taken into account by the citing commission, Pape propels himself straight at Heaslip's back and has no intention other than making contact with Heaslip's back. The two incidents are incomparable. McGrath got a yellow, both the ref and TMO thought this was the right decision, no one thought it was a red at the time and in real time the incident looks almost harmless. Yellow and no citing may have been a tiny bit lenient and a red may have been deserved, never worthy of a ban though.<iframe id="rufous-sandbox" scrolling="no" frameborder="0" allowtransparency="true" style="display: none;"></iframe>