“A priestess of Avalon does not lie. But I am cast out of Avalon, and for this, and unless it is all to be for nothing, I must lie, and lie well and quickly”
Irene does not stay to watch the fall, though she knew it would come. But she'd also seen what Moriarty had never noticed, never grasped. That great heart, indeed. She smiles, escapes and finds herself on the other side of the world, blending in to the sun and the crowds and keeping half an eye on the news.
She'd seen - seen the way he looked at John, the affection for Mrs Hudson, for the lovely Inspector Lestrade and his daughter, even for Molly. Irene had seen and Moriarty had not, because to him he and Sherlock are entirely the same except Sherlock has sentimentality. What he never knew, Irene thinks was that the great detective might have a great heart but his vengeance and defence of those he loved was deep and long.
Sherlock Holmes was never one of them. He would do what he must to allow them to remain alive, even as it broke his heart and she'd known it from the first moment he'd looked at John in her presence. After that it was easy enough to play the long game, to come out alive.
Sherlock never deduced her long game, after all.