I agree Sherlock came to admire Irene for her intelligence, cunning and gamesmanship - she's a worthy opponent - she certainly wasn't boring! Was he in love? Not likely (yet), sentiment is chem defect on the losing side, but I like to think he was finding himself challenged and perplexed that he might actually be experiencing this defect - because something about Irene had sparked it.
I think both of them started out being intrigued by one another's game, the intrigue eventually becoming admiration and respect, and heavens! leading to sentiment. Irene revealed her hand first literally - but Sherlock's actions in assisting her in Karachi - reveals his, perhaps. I can't imagine it was all to fool Mycroft!