Although I have seen
Noel used on females,
Noelle is the female version of
Noel. Both can be pronounced No-El, or
Noel can be pronounced NOL.
I wouldn't think it's a big deal if you used
Noel for a MN on a girl (by the way, I love the name
Saoirse) because not a lot of people will see it, and I suppose a lot of people these days are using boy's names on girls anyway, not that I condone following that trend. :)
**
Vita Brevis, Ars Longa**