It is being pronounced in a Munster dialect, I believe. Wondering myself why you have to write 'm'fhón', instead of 'mo fhón'??
It's not that it acts as a vowel, so much as it isn't pronounced at all.
A bit of history: where we now use the letter 'h' to mark lenition, we used to use a dot above the lenited consonant. This dot came from an old scribal practice used by monks to mark a letter to be deleted. This was generalised from letters like 'f' which are deleted when lenited to mark lenition in general.
After posting, I later thought that it was the "h" sound that made the difference. But I think it's really funny if a listener didn't catch the "h" sound. Thanks for your response.
I'm not sure how to explain the grammar exactly. However' the verb "déan (rinne)" is being used as a "briathar cúnta" here. "Dhearmad mé m'fhón" would be a direct translation. "M'fhón" is the direct object and there is no "ar" required. But when "rinne" is used, "m'fhón" is no longer the direct object and we always say "rinne mé dearmad AR rud éigean".
thank you so much for taking the trouble to reply; it has been a great help and I think I shall always remember that now
For anyone reading this since 2016-05 or so, a Connacht dialect is now presented.