After my second viewing, I figured that the only weakness Irene had, was that she thought she finally met her match. What she did not consider was that Sherlock is able to switch his heart completely off. That's how I saw it anyway. On the plane, from the moment he realizes Irene betrayed him, Sherlock doesn't yell, he doesn't accuse her, he doesn't plead with her. Instead, he begins to think, trying to connect lines, analyzing everything Irene did so that he can beat her at her own game. It reminded me of the scene in The Great Game where Sherlock tries to figure out what is wrong with the painting. Everyone around him freaks out at the thought a child might get killed - he blocks that all out and tries to solve the puzzle.