This question is pretty much directed solely at Bríd. So I was talking with a friend of mine from Carraroe, and he used uaidh an bhainisteoir. this isn't the first time I've seen it (in fact, Gumbi and I discussed it once after someone posted it on /r/gaeilge). Is this common? Does anyone know how it came about? I just find it super interesting.

It does exist in other dialects, I guess, and in Manx, and there are other prepositions that have been replaced by the 3d singular masculine pronominal form.
Actually, "faoi" was "fo" but it has been replaced by "faoi" (which means "under him"), "lesh" in Manx, has replaced the equivalent of Irish "le", "fríd" (Ulster) has replaced *"frí"... There are similar things in Sc. Gaelic and in Manx.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Thanks Lughaidh. I knew that ó was used as uaidh because it's mention in An Teanga Bheo: Gaeilge Chonnachta but I had seen no mention of uaidh an used like that. So is it used in Donegal as well?

At least it exists (uaidh instead of ó) in Rosguill Irish -- see Grammar of Ros Goill Irish, Co. Donegal, L. Lucas, p. 180.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

I don't know really. I'm sure Lughaidh has explained it well.

All I know is that we do say that, pronounced Y. And I did a soundfile a long time ago here-
viewtopic.php?f=28&t=1095