Part of John 9:25 (ABN)
"tá a fhios agam aon ní amháin go raibh mé í mo dhall ach anois go bhfuil radharc agam"

Douay Rheims:
"one thing I know, that whereas I was blind, now I see."

In the Irish, why do we need both aon and amháin??

Thank you

You either need to use the structure aon X amháin, or just simply X amháin. Thus you could say aon ní amháin, or ní amháin. You couldn't say aon by itself because then it would carry the sense of 'any', i.e. aon ní = 'anything'

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar