Two tailed siren fresco, Pfarrkirche Göriach / Gorje (Mariä Namen), municipality of Hohenthurn, Carinthia, Austria. (English: Göriach Parish Church.)
Some siren frescos survive unscathed throughout the centuries; some are damaged. Unfortunately, it looks like this particular fresco was painted over at some point, and then later restored. In addition, this is the only photo I could find of her… and this photo is from a 2015 blog I happened to save as a pdf that’s now offline.
So let’s look at what we do know: the church was first document in 1312 or 1316. It was destroyed in 1478 by the Turks, and was rebuilt between 1489 and 1516, according to the church's website. The church is located in Carinthia, which historically had a large Slovenian-speaking minority. I think this is significant, because the biog I originally found her on was in Slovenian, and referenced Faronika the fish, a two tailed mermaid popular in Slovakian church art. The other two tailed siren in Austrian church art I’ve found so far is also in the Carinthia region.
While faded, she clearly has two tails, putting her into the "sirens in church art" category, and seems to be wearing a red shirt or dress. I’ve seen some images of Faronika also wearing a red dress, such as The Church of St. Margaret in Turčianský Jaseno, Slovakia:
I have a book that I'd like to look up, when I can get to a well stocked academic library: Göriach, Gemeinde Hohenthurn, Pfarrkirche Mariä Namen, Friedhofskapelle, Pfarrhof. In: Die Kunstdenkmäler Österreichs. Dehio Kärnten 2001. S. 217–218. Until then, I'm calling my research here.