I don't know Taylor Swift. I don't know a single solitary Kennedy.
Here's what I suspect happened:
1. Conor was invited as a cousin of the bride.
2. Conor didn't RSVP because very recently his mother died and there was a great deal of angst going on about all that and her two burial plots and so on. And because she wasn't around to remind him that he NEEDED to RSVP.
3. Conor calls a couple of hours before the wedding and asks if Taylor can come with him.
4. MOB says no. They didn't expect him and they don't have seats (food) for them at the reception.
5. Bride says "No, cool! Of course you can come. I'd love it!"
6. Taylor and Conor attend wedding. Go somewhere else after (while people at the reception are eating). Return to the reception for the dancing part.
That's my version as someone who wasn't there, saw nothing, heard nothing and knows nobody, and I'm sticking to it. LOL!