Feacht n-aon means "on one occasion". Does anyone know the history behind the n- here?

I’d suppose a petrified accusative case.

Thank you. I see now.

In terms of modern usage, I've only seen this used to start fairy tails.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Thank you. I see. A kind of "once upon a time" thing. I've found it on p45 of Quo Vadis? by Aindrias Ó Céileachair. This book is my latest project and contains many old things/archaisms.

Yes exactly:
"Feacht n-aon a bhí bean ag maireachtaint...."

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal