Yes, that is exactly what I was asking about! I hadn’t seen your ILF post (I don’t think it was there when I asked this question), but from that I gather that it disappeared somewhere in the shift from Classical to Modern Irish?
I guess you’re referring only to the third singular? Cuz of course many of the other persons/numbers still exist in Munster or in e.g. Connemara in echoic position.
I understand about the echo form usage; “fell out of usage” was careless terminology on my part. I was asking when the 2nd and 3rd person, singular and plural, present tense endings [-(a)ir, -(a)idh, -t(a)i, -(a)id] became the same as the habitual present tense endings [-(e)ann].