phoney and anus

phoney is most likely an alternation of fawney "ring", a borrowing of Irish Gaelic fáinne "ring". A fawney rig was a swindle involving a fake ring, and has been in use since the middle of the nineteenth century (The Origin of 'Phoney', Peter Tamony, American Speech, Vol. 12 no. 2, pp. 108-110).

Irish Gaelic fáinne was an alternation of áinne, from Old Irish ánne. The Proto-Indo-European root is *āno- "ring", which became Latin ānus "ring, anus".

McBain's has

fàinne
a ring, Irish fáinne, áinne, Old Irish ánne, *ânniâ; Latin ânus, English annular.

The Early Irish Glossaries Project in the entry for áinne "ring; circuit; anus" has "Cf. fáinne", but no word on how they are exactly connected.

Where did the /f/ come from? I know Celtic languages have a habit of losing initial /f/, but this one was added.