The "oi" sounds really different between oibrím and oifig... Is it just something I have to memorize?
It's her dialect.
Before it said "isteach" means in. But we also have "sa" and "i". What are the differences between the three?
So, i and sa are prepositions. sa is i + an (and becomes san before vowel sound) and causes lenition. isteach is used to describe motion. I suggest looking at the FGB samples