Can a preposition really be the last word in the sentence? I know it can in Germanic languages like English, but in Irish?
It should be air in indirect relative clauses (and leibhéal is masculine). Or you can use cén leibhéal ar a bhfuil tú. You can see this here under "indirect relative clause".
For present tense:
After the verb:
antecedent + a + verb + subject + preposition + pronoun
Before the verb:
antecedent + preposition + a + verb + subject
Both are accepted. Notice that in "after verb" you have it "preposition + subject", which does inflect.