Having Irene be Moriarty is an interesting take, I'll give them that, and the actress plays her well. That being said, I felt the ending was rather weak. Moriarty is supposed to be at the very least Sherlock's equal intellectually, and also works through proxies, has an entire "organization" of them. She didn't need to stop the assassin herself, but I can let that slide, since she decided to reveal herself to Sherlock. But then, she takes even more stupid and unnecessary risks. When she grabs Watson in the middle of the street in broad daylight, not to mention delivering the handgun to Narwhal herself. Makes no sense. Then there's the whole trap thing. Even though Sherlock didn't prevent the murders, he still solved the case. He wouldn't OD or even get high because of that. He loves a challenge. As soon as I saw him lying on the floor I knew what was going down. And then she shows up at the hospital, why? I'm sure she could have found out that he wasn't dying or whatever without actually going there, and if she really wanted to see him before she left town she could have set up a less risky way to do it.
And now she's been arrested. Surely someone who's so smart and resourceful with an organization of henchmen can orchestrate a prison break without even trying too hard. Which I'm assuming will be what happens next season. Unless they go the other route, and have her sit in prison while some other plan of hers is set in motion. And Sherlock visits her, and she tells him going to prison was part of her plan, blah blah. Hope I'm wrong, but this season finale doesn't give any indication of that.