I'm compiling stuff I learn about their culture and collecting all the books I can find on it.
I've been able to make a lot of intuitive guesses about Celtic cultures that turned out correct.
Ive got a Discord for researching Celtic culture, but with emphasis on Irish Celts, but I haven't worked to get it active yet.
I kind of prefer to store specific info there and turn it into a *library* app. I know it can be done.