Hi! I’m working on my prepositions some more, and I’m having difficulty finding an Ulster pronunciation for “aige/aici/acu,” I’ve put them through the Abair.ie synthesizer, but it’s hard to tell which syllable the stress is on. I also rewatched “Now You’re Talking Irish” Episode 20 on YouTube, but again, it’s hard to tell which is the stressed syllable among all the speakers. Also, the “e” in “aige” sounds like an “uh” sound sometimes and an “aa” sound other times.

Can anyone help? Thank you if so!

The stress is on the first syllable.

Check this: http://www.fuaimeanna.ie
Click on the second yellow box, which takes you to sample pronunciations. On pages 3 and 4 of 77 you can hear 'ag' in all its pronoun forms. The Gaoth Dobhair woman pronounces the second syllable of 'aige' as 'ah'. However, I hear it more commonly pronounced by Donegal speakers as somewhere between 'eh' and 'ih' - closer to 'eh'. Not unlike my pronunciation of the first syllable in "heaven" or "ever".