It is a wonderful name! It's rare, indeed. My name is Mirela, the Romanian form and back in the day I was so avid about finding out what it meant and I asked one of my high school teachers about it and she said: "It comes from mirar, the French word for the English to admire. It also means the marvelous one."
I've lived in France for the better part of a decade and have never once met a Mireille. It could be that its origins in Provence make it a 'peasant' name, equivalent to the American 'redneck', so any name that originates there will NOT be fashionable here in Paris.
I live in the US and my name is also Mirelle. When I was born my parents named me after a French singer named Mireille. When I was about seven they changed it to Mirelle because it was easier for people to pronounce.