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I actually wonder why Sherlock is so sure of this and why the terrorists (or whoever they are) are so certain to kill Irene. BECAUSE the Holmes brothers felt sure of this, they let her go pretty much unconditionally, so it wasn't like Irene had to testify against someone or lead the British government to the cell, in exchange for leniency?
I know with these kinds of villains it's common for them to fall out and for one to kill the other for "failing," but why had Irene failed? What was to stop her from going right back to doing...her thing...once the Holmes brothers let her go?
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Good question. Not sure why no one has addressed it yet.
My reading is that she will be executed because she failed, and because all those secrets she had on her phone are now in the hands of the British government. Also, her phone was her protection - she had so much dirt on everyone that no one dared to come after her. Once her blackmail material was gone, she was fair game.