According to several well-qualified teachers of contemporary Irish, the 'f' in 'féin' is pronounced as an /f/ only in a very few specific instances: > When 'féin' is used as a prefix as in 'féinmhuiníneach' > In the name of the political party Sinn Féin (but not when the uncapitalized phrase 'sinn féin' is used to mean 'ourselves'). > Or when the word 'féin' is pronounced by a single, very small speech community (where, I don't know) of no more than a few hundred speakers Everywhere else and in all other contexts, it is pronounced as an /h/. I would guess that the variation you heard is doubtless one arising from uncertainty on the part of the speaker. There is no grammatical cause for it.

It's mostly to do with dialect. In Ulster "féin" is almost always pronounced "héin" (and in fact many Ulster Irish speakers write "fhéin" to show this - though this spelling isn't permitted in the Official Standard - and this trend also continues up into Gaelic Scotland where "fhèin" is also pronounced "héin"), although there are some instances where the "f" is pronounced "f", most notably in "cheana féin" ("already"), and when used as a prefix (as mentioned by Dáibhéad). Down in Munster, on the other hand, you're more likely to hear "féin". You possibly heard these phrases said by two different people with different accents or dialects; I myself would certainly say "héin" in both cases.