"Leth uair an dèidh ochd."
Translation:Half past eight.
That's a tough one, for sure! -idh can give it a little bit of a longer "ee" sound at the end but I think conversationally they're going to almost always sound the same :(
I can't tell a difference between them, either, regardless of where they're being pronounced (they sound identical to me at https://learngaelic.scot/dictionary as well, and they usually do a really good job of exaggerating certain sounds for learners).
If anyone else has any tips for distinguishing (other than, as you noted, context), I'd love to know as well!
I know it doesn't work on Duolingo, but I find the most reliable clue is the prosody - that is the rhythm of the sentence. An-dè is usually followed by a comma or full stop. Even if not, there is a least a tiny pause. But An-dèidh has to be followed immediately by a noun phrase and they usually sound run together.
And next winter you will hear an deigh 'the ice'.
Because this is not normal modern English in any dialect, so far as I am aware. When I searched online for half past eight o'clock I got a load of links to sites telling me how to tell the time in English, and they mentioned eight o'clock and half past eight but none mentioned half past eight o'clock. The only time I would expect to see or hear it is when someone is being deliberately pompous or old fashioned, such as on a printed wedding invitation.
The English here corresponds with the Gaelic - it would be equally weird to say leth uair an dèidh ochd uair.