I'm wondering if "behind" (thiar) follows the same pattern as the other prepositions. I mean, "before" (riomh) is there, and "over" (thar) is almost the same word...could thiar be construted thusly?

thiar:
thiaram thiarat thiaris thiaristi thiarainn thiaraibh thiarstu

it seems to me like a no-brainer, but I cannot find "behind" on any list of prepositional pronouns. is there some reason "before" should be a prepositional pronoun, but "behind" not?

Thiar is an adverb, not a preposition.

Behind me: taobh thiar díom

go raibh maith agat (all i had was the single word added under the list of prepositions from an old class)...

however, if "before" is a preposition, what is its equivalent opposite, "behind" or "after"?

roimh - before. tar éis, i ndiaidh - after.

I think there is no preposition which is exactly equivalent to "behind" or "after". The phrase will very depending on the exact context.