as the title reads whats the difference between "in aice" and "in aice le"? when do you use one over the other and how does it affect the grammar of the sentence?

I don't see any difference in meaning. It just depends on the dialect.

In grammar, the difference is that "in aice le" is followed by the dative case, and "in aice" is followed by the genitive.

"In aice leis an bhean/mbean"
"in aice na mná"

"in aice liom"
"i m'aice"

"in aice le teach"
"in aice tí".

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third option for this one "In aice leis an mnaoi"



I am not aware of any different in connotation or usage. If there is one I am interested as well in hearing it.

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GRMA

Or leis an mhnaoi (bhig, bhoicht…) – as it should be in dative. In Sliabh na mBan bhFionn both – an mnaoi and an mhnaoi – can be found.

Judging from leamh.org, an mnaoi would actually be accusative in early classical Gaelic (but accusative and nominative merged a long time ago, so I don’t believe anyone today would say eg. chím an mnaoi or rugadh an mnaoi sin…), and I have no idea what form would adjective (except for gcaoil given in examples there) take in that form (an mnaoi mbig?).

recently in reading I came across "don mhnaoi" and "leis an mnaoi" but I don't know more than that. Unfortunately neither had any adjectives...

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"Cén t-ainm atá ar an mnaoi sin?"

Just yesterday I heard this from a native speaker from North Mayo, a man in his late fifties. (Though of course, he said "mraoi")

Agus cén t-ainm a bhí uirthi?

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