Are epenthetic (i.e. unwritten) vowels used in the usual places after prefixes? Thus, are garmhac, bunbhrí, an-chiúin, etc. pronounced with an unwritten "uh" between the prefix and root? Bríd pronounces garchabhrach without it, but I just wanted to make sure.

You’ll have to ask a native or advanced speaker about this but I would imagine it happens because it’s easier to say things that way. In the case of “an-“ you do see the variant “ana-“.

I don't think there are epenthetic vowels in these words (except in "an-chiúin" in Munster since they use "ana-" instead of "an-" all the time).

/garwak/
/bɔnv'r'i:/
/anx'u:n'/
/garxo:rax/

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

You can also hear the word here:

https://www.teanglann.ie/ga/fuaim/garchabhair


Some words, like the name "Sorcha", have helping vowels added after the first syllable. Maybe that is what you mean. I don't know the rules of when that's added, you just kinda know from experience.

"ana-chiúin" - I think that's only Munster usage.

Aye, there's a rule that says what are the pairs of consonants that get a helping vowel between them, but that rule normally doesn't apply when there are prefixes, or with compound words.

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

Alright, thanks, the precise rules are actually spelled out here (to which I've contributed myself); it's a little technical, but it should tell you exactly when it occurs.

Bríd is right. "ana-" is used in Munster. They also have "bun-, buna-" but in the sense of "a little, somewhat" like "yellowish" > "buna-bhuí". I can't find any other examples where you have an option of include an epenthetic vowel.

And it is seana-, e.g. seanabhean, in Munster.

One more thing: is it mol(a)far or just molfar? I'm guessing the latter because verb endings are distinct morphemes.

I'm pretty sure they say "molfar" in Munster, without the helping vowel.

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