Phrases of irreality require the subjonctif. If we now have a very closed look into the phrases it is less the pure irreality that we are talking about, but more the feeling of irreality of the speaker phrasing the sentence.
il est impossible = it is impossible
Il est impossible qu' il le sache.
It is impossible that he knows it.
il est hors de question = it is out of question
Il est hors de question que nous payions pour pouvoir respirer.
It is out of question that we pay for being able to breathe.