On y aurait cru
I translated this as 'we would have believed it' but I am not sure. Does anyone have a correct translation?
Google translate suggested we would have been believed but that seems even more wrong than mine although I could be wrong on that too
"On y aurait cru" has "y" replacing "à + something mentioned before, like "on aurait cru à cette chose".
"Croire à quelque chose" is to believe in something's existence or reality.
"On y aurait cru" means that we would have believed [something mentioned before] was true. So I think your translation is fine, unless context would reveal something different.