> I thought 'fofaicfe' was the 3rd singular future of 'fo-ácaib' with a
> weird reduplication of the 'fo'--a Middle Irishism, perhaps?
It is not a reduplication, but yes, there is a strong tendency
throughout the Irish language history that "wrong f's" start to crop
up at the beginning of words with vocalic initial. This is due to the
fact that after a leniting particle a word starting with a vowel
could always be reinterpreted as having a lenited "f". So, when you
used the word outside of leniting contexts, you could stick an extra
cool-looking "f" onto it.