When I first started learnign Irish I learned "next to" was a combination of "i n-aice le" i n-aice leis an bhfuinneog, now in Gaeltalk and even looking in ó dónaill its just "in aice" in aice na fuinneoige.

is the difference simply

In proximity (with) the window
and
In proximity (of) the window (window gets put in the genitive)

Gaeltalk are right on this one.

In aice ruda - besides (or compared to) something

But PUL has "cimeád in aice le duine" to keep up with someone, eg in a race. Possibly the "le" is in this phrase through confusion with cimeád suas le duine, in itself formed literally from the English "keep up with".

also, whats the difference between the above and using "cóngarach" and "cois"

Ive wonderd if they are all interchangeable

just as in English

Next to the fire
Beside the fire
near the fire
in close proximity of the fire (if ya want to be fancy)

all mean the same pretty much.

Go raibh maith agat a David

'In aice le' is perfectly fine, just as 'in aice + Gen'.

In aice leis an gcéibh, in aice an tigh, in t'aice (= i d'aice) are mentioned in the Morphology of CF Irish.

Of course, there is also 'in aice + Nom' in the vernacular ('in aice an fharraige', etc.).

cóngarach do - near

i gcóngar do - close to

lem chois - besides me, along with me.

le hais - besides

achomair do - near/close

buailte leis - close to him, almost upon him

i leataoibh an dorais - besides the door (leataobh: one side)

ina ghaire - near it

gar do, i ngar do - near

lámh le - close to, hard by

i dteannta ruda - near, besides, along with

suas le - close to, up besides, near

I don't think "in aice le" is 'just fine' in Cork Irish, other than in the one phrase "cimeád in aice le duine", which seems to be some sort of Béarlachas. And Cróga is learning Cork Irish.

Thanks David, I didnt realize there were so many ways. Looks like I have another list to add to Quizlet.

Oh! Typo alert! I meant "láimh le", not "lámh le".

Don't forget "le hais le" /l'ahɪʃ l'eː/ as well as "le hais".