The real difficulty is that there are several words for "time" in Irish, and they're not always interchangeable. But now that we know the context, I would go with aon lá amháin, aon uair amháin as originally suggested.

You could also have aon lá amháin, aon shaol amháin (only one day, only one life/world)

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Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

That would need to be aon saol.

DTS don't lenite after DNTLS.

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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).