I need air

This preposition is both simple and annoying. Let's start with the simple:

mi	thu	e	i	sinn	sibh	iad
orm	ort	air	oirre	oirnn	oirbh	orra
[ɔrɔm]	[ɔRʃd]	[ɛrʲ]	[ɔRə]	[ɔːRNʲ]	[ɔrʲɪv]	[ɔRə]

Yes, oirre and orra are pronounced the same way because there no longer a disctinction between broad and slender rr in modern Scottish Gaelic. But that's only very rarely confusing in context.

Air, lots of air

Unfortunately, yes. You'll most likely first meet air meaning "on" like air bòrd "on board" and be told that there's no lenition after air. Yay, you think. The you run into deich air fhichead and you just file it under it's a number, maybe a special case. Then you bump into beag air bheag and maybe it looks a bit strange but you just learn it as a phrase. But as you progress and run into air chor-eigin and air bheag airgid you begin to wonder about the "no lenition", not to mention that many of these don't really fit the translation of "on".

Your confusion is justified. What makes this so confusing is that what looks like air is actually the result of 4 different words all ending up with the same spelling. Don't worry, this isn't going to be as bad as The many functions of a.

Work in Progress