"Are you scared, friends?"
Translation:A bheil an t-eagal oirbh, a chàirdean?
The noun meaning fear is eagal. But this is a masculine noun, and the definite article an the prefixes t- to masculine nouns starting with a vowel, so the fear is an t-eagal. And in such sentences like this in Gaelic (when you speak about one’s feelings and emotions) the definite is typically used (you say literally that the fear is on you), that’s why it’s an t-eagal here.
t-eagal on its own (without the article) doesn’t mean anything.
When addressing (multiple) friends you have to use plural. It’s not formal in this case, it’s just the form used for multiple people.
Like in older English (or even in some modern northern English dialects) you could say art thou scared, friend? to one person but never to many, you also could say a bheil an t-eagal ort, a charaid? to one person, but you have to use the plural are you scared, friends? a bheil an t-eagal oirbh, a chàirdean? when addressing multiple people.
As for falabh, deareg, also Alaba for Scotland, etc. – you are absolutely right! This is a common phenomenon in Goidelic languages called epenthetic vowel. See Svarabhakti or The Helping Vowel on the Akerbeltz Wiki, epenthesis in Scottish Gaelic phonology on Wikipedia and Epenthesis on Scottish Gaelic Grammar Wiki.
That’s also the reason why many Irish and Scottish English speakers pronounce film more like filəm, etc.
But I don’t think this happens in càirdean. The epenthetic vowel is not inserted before d, from Akerbeltz:
Mostly this happens when you get l n r and /ʃ/ coming together with b, bh, ch, g, gh, m, mh.
In càirdean you’d rather hear slender s inserted between r and d (so as if written càirsdean).
Aren’t you confusing the older form càirdean (used in vocative here) with the newer plural caraidean (with broad r and short a, formed directly from singular caraid, not used in the vocative) – which has an additional non-epenthetic vowel between r and d?