Thank you Lars. I think I misremembered where Ó Siadhail said that form was from.

But I think ar bheith do and all similar phrases can be overdone, or have a non-tigh-tábhairne feel to them (neamhthightábhairneachas!). Dónall Bán Ó Céileachair objected to the sentence published in Aodh de Róiste, a translation he had done, but published by An Gúm: "Is dó’ liom nách beag san", arsan t‑uachtarán ag stad do. He complained in the quarterly journal An Músgraigheach that this was not his Irish, and he it should have been :.... agus do stad sé.

Maybe PÓS meant to say "fosadh".