From elsewhere, Domhnaillín said:

Complex prepositions (those consisting of more than one word) work differently, e.g. inár dtimpeall "around us", os do chionn "above you".

Regarding

os cionn = above
os do chionn = above you

Would it be accurate to say that os do chionn translates literally as "in your aboveness"? I know that's crappy as a translation, I'm just trying to get a feel for the foreign logic at work here since it's plainly so different from English.

That example applies to space, but I remember encountering something similar with regard to time -- i mo dhiaidh, "after me".

Os is an earlier simple preposition meaning "over, above" and cionn is a dative form of ceann now only found in fossilised expressions (e.g. thar cionn "excellent"). So the literally meaning is "over your head".

In a similar fashion, you could rephrase "i mo dhiaidh" as "in my wake", "in my trail", "in my aftermath", or something like that. Not an actual translation, but it does clarify the logic at work there, as you say yourself.

I think that all compound prepositions originated as combinations of simple preposition + noun, and that explains their behaviour.