I really like Mirabelle, it is a name I have never really thought of. I think you are safe from it becoming too popular. I think a lot of people naming babies right now are staying away from the Isabelle, annabelle names because the belle and bellas are becoming way too popular. So I think this name has slipped by without catching on and you are clear of a name-demic :)
I think you are quite safe with Mirabelle. it's one of the 'under the radar' type of names that is pretty and individual at the same time.
All the best!
Mirabelle isn't even in the top 1000 in the US, so I think you are safe.