The password is in the game files after getting the letter.
If you need help getting the letter, in the chapter where you meet Aífe, make a different choice.
Viewing post in The Good People (Na Daoine Maithe) comments
The letter is a picture in the game files that appears when you unlock it. The message that is circled is the password that you need to translate from Ogham (you just have to translate the letters). It is read from bottom to top. Here is the alphabet you can use to translate it: https://omniglot.com/writing/ogham.htm
It sounds way too high tech for me. I keep seeing comments about passwords but I don't even get what it has to do with a visual novel.
Edit: Nvm found the files and it let me type after I opened the game another time, but I can't get the password right even with the code, not sure how I'm messing it up so badly :'( Oh well.