According to what I see in FGB, they put a hyphen:
- when the 2nd element begins with ch, so that you don't get chch in the word,
- when the 2nd element begins with a vowel that might be pronounced unproperly otherwise : droch-araí, which is /'drox 'ari:/, but if it were spelt drocharaí you'd pronounce /'droxəriː/.
Btw all the words you've written in your message, have no hyphen in FGB: drochíde, drochbhlas, etc.
I don't know where you saw them with a hyphen, but either they were in pre-1950 texts, or they contain mistakes...