I guess you might be encoutering a Duolingo software problem, where the difference is so minor that it treats it as a typo and accepts it, instead of treating the wrong pronoun as an error. Unfortunately I don’t think the contributors (who get and review the reports) can do anything about it. This would need tweaking the Duolingo typo-classification algorithms.
Yeah, I do too. And it doesn't help that duolingo doesn't even mark it as a typo (greannach should have two Ns).
Also, I use the Android Google Keyboard feature, and it has grianach but it doesn't have greannach so it keeps trying to put in Greanadach - I don't even know what that means yet. (It's always capitalized so maybe it's a name). There's a bunch of words it doesn't have so I'm always getting bad suggestions. It's both helpful and annoying at once.
Agreed that they differ in the first syllable in their vowel. But I believe the /ia/ in grianach /ɡr´ianəx/ is a diphthong, so it still scans as a single syllable, and in greannach it’s perhaps /e/ in some dialects but AFB gives /a/, so /ɡr´anəx/. Both words having the same syllable count – just the first syllable of grianach starting with clear /i/-quality and turning into /a/, while greannach has a single vowel quality (/a/ or perhaps /e/) throughout the whole first syllable.
Also, the exact quality of the n-sound might differ (lenis [n] in grianach, fortis [n̪ˠ] in greannach – but this is harder to hear and not sure how consistently it is differentiated across dialects).