In Polish, this name etymologically isn't a combination/compound of Maria and Anna, but rather it's the feminine form of Marian (kind of like how Julianna is the feminine of Julian in Polish), and it was historically a very commonly used name in Poland because the name Maria was sacred and reserved only for the virgin Mary, i.e. you were not supposed to name a baby Maria, so instead Marianna was used. Source: https://deon.pl/imiona-swietych/marianna,5954
Marianna is also Finnish, Latvian, and Estonian. The name day for Marianna in Finland is August 15. The name day for Marianna in Latvia is September 20. The name day for Marianna in Estonia is September 8.
I really love the sound of this name. I've been searching for a Pen Name, and I really liked Arianna, but I wanted something that started with an M so I searched this name and was so glad it showed up.
The name Marianna (my sister's) is also the name of a blessed woman (not as high as a saint in Greek religion but of great importance as well) and it is celebrated on February 28 in Greece.