Hiya
I was always under the impression that Mona was Old English for "The Moon" and I was surprised to find the entry on here does not reflect that.
I found this entry
https://en.wiktionary.org/wiki/mona#Etymology confirming this in Middle English, Old English and Old Frisian.
The source that website uses links to
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED28542