I didn’t realise the prototonic future had lived on any longer than early Middle Irish in this verb. I thought the only future in existence was the regularly formed f future based on the deuterotonic present stem.

Do-ghéanfaidh makes very little sense historically, doesn’t it? Are newer f futures not normally based on present stems, rather than old future stems?

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Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

Ghéanfaidh is the old -é- future, and the -f- ending was added by analogy with the other verbs (the Classical Irish form was do-ghéana, see Stair na Gaeilge p.412).

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Thinking about it, this isn’t so strange after all. I’m not sure why I couldn’t think of parallels to the productive f future being added to a future stem—it’s exactly the same as what happened with béra and ísa, for example. Duh.

_________________
Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.