High Fitz,
It's interesting to compare the word endings "iadh", pronounced "y", as in "my", and "adh", which is silent.
So, Fasadh could possibly be pronounced as "Fasay", or "Fasa", and "Fiadh" could be "Fiay", or "Fia". I think the latter of the two pronunciations seems likely, due to more support for that in what I've read.
Literally translated, "Fasadh Fiadh" means "the level spot of the deer", or "the stance (or station) of the deer" which I presume to be "the plain of the deer", or "the place of the deer", which is more likely. Makes sense to me. Hope this helps.
Regards,
TE.