> "I will cry my fill, but not for God, but because Fiond and
> the Fíanna are not living."
My first guess is that it's found in "Acallam na Senórach".
If you can locate it in the translation _Tales of the Elders
of Ireland_, that would be a great help. The language,
incidentally, would not be OI in that case, but rather largely
Early Modern Irish with some MI orthography and other touches.