I completely agree with you, that was some of the best acting I have seen on the show for a long time, precisely because it was so simple. Any one who has been to acting classes will tell you it is actually easier to do the loud and in your face stuff, but the simple and subtle is much more difficult. That was excellent. All of the nuances of the confusion and the contradictory emotions Irene was feeling were captured and portrayed simply and effectively. No big histrionics, just raw honest emotion.
I am really looking forward to this story and I do hope it isnt one of those which is over and done with in five minutes like so many have been. That would be a criminal waste.