I think Irina became "the impact character" the same way Grace was. I feel that Irina dying made Amy & Dan want to win the right way. The same thing happened at the end of Storm Warning.
An impact character in a way makes something happen and dies when their usefullness has run out. The 39 Clues has had it flaws. Irina dying was definately wasn't one of them. Irina surviving would have brought the downfall of Isabel Kabra earlier, and the series is must less epic. I made a fan thery about Irina being a Madrigal.