I remember reading once that "f" used to be pronounced with just the lips in Irish, rather than by the upper teeth engaging the lower lip. Does anyone know if this is still the case among native speakers, or has it fallen entirely out of use?

I know that, in Ulster, this is probably what caused "f" to morph into "p" in some words (e.g., "fill/pill").

Redwolf

I was taught early on not to bite my lip in the Irish f. I think the f is still somewhere between an f and an h in Gaeltacht Irish. It may be part of the reason féin sounds like héin in some of the dialects.

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

yeah f (and slender bh and mh as well!) should be bilabial in older speakers' speech at least. For young speakers I don't know. It may depend on the way their family speaks.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne