Technically, you are the eye and the mirror at the same time, the mirror just does not reflect 1:1.

As for "how one comes to know the unknowable" -- you simply must observe yourself and discard everything observed as observed and it will eventually hit you. It cannot be figured out. You can, however, figure out undivided existence which also reveals the non-dual nature of reality (i.e. observer, observed and observation are one and the same). In religious terms one could say that God Gods God's God, but such communication would not get us very far :D

Well, perhaps the best way to describe it is self-realization (as opposed to self-knowledge).