I See Fire

Interesting question.

This is all just speculation on my part, but here's my brain dump on this.

1)
A lot of the songs were arranged differently than previous albums. They were definitely going for a bit more traditional Irish album. Having her sing alone with minimal accompaniment was consistent with that theme.

2)
This was arranged as a quiet song (as was the original version). A choir might have been counterproductive to the sound they were going for.

3)
I thought for a bit that they just wanted to switch things up a bit by having a pure solo performance. However, Isle of Innisfree (Rebecca solo) was right after I See Fire on the DVD, so this theory doesn't really fit.

4)
The choir didn't have any significant costume changes so I don't think it was a break in order to change outfits.

5)
Maybe the producer wanted Mairéad to be the focal point because it was her first special with CW.

6)
Mairéad is awesome and she can carry a song herself.

Personally, I think subtle choir backing in the second half of the song would have worked well. Nothing to overpower or drown out Mairéad's beautiful voice, of course, but just a touch of choir would have been fine.

I'd be interested to hear other thoughts.