Yeah it’s obviously not “an t-aon” for genitive. Sources seem mixed on whether for déag it would be “an aon” regardless of gender, “na haon” for feminine, or “na n-aon” regardless of gender. The latter makes sense given that the “déag” numbers are treated as plurals with respect to their adjective declension, unlike the “aon ___ is fiche” numbers, which are treated as singular. I have come across this:
In imeacht na n-aon bhliain déag dar gcionn
(source is http://www.militaryarchives.ie)
but not sure if it is written by a native or not. The rest of the text looks to be accurately written.
But as you say, it seems that it’s something that is either not said or said so infrequently as not to warrant teaching it.
