Ní bheadh a fhíos agat = You never know

I had guessed it meant "You wouldn't know".

How would you say "You wouldn't know" in Irish?

I would have guessed that Ní bhíonn a fhíos agat = You never know

I think the "would" is implied in the English phrase. And the Irish translation you have doesn't include the "never".

Ní bheadh a fhios/'fhios agat riamh would be a literal translation, but it doesn't sound great to me to be honest, but it's workable I think.

I agree with Gumbi "You never know" is really an English idiomatic way to say what would be "You wouldn't know" in correct English.

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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or just 'b'fhéidir'!

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And to say "_you_ wouldn't know" (because you weren't there, etc.) in a "shut up and mind your own bloody business" tone of voice, probably:

Is dada/tada/faic a fhios a bheadh agatsa

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).