Is it right to manipulate irrational people? It is a very interesting question.
Firstly, the actor (the one doing the convincing) should ask himself what his true intention is in convincing the subject (the one being convinced). Many people will try to convince others, not because they believe it is best for the subjects, but because they are trying to mend a personal insecurity. For example, I may try to convince people that the Earth is flat, not because I believe it is in their best interest to believe that, but because the more people I manage to convince of a flat Earth, the more I feel legitimized in my own belief. In this case the actor is merely using the subject as a means to a selfish end.
If the actor does not have the subject's best interests at heart, his actions are unjust.
Secondly, the actor needs to be very certain that he knows what is best for the subject. And this is clearly very tricky. If the actor is successfully convincing a subject, he is exercising power over that subject. With this power comes great responsibility, since changing a subject's view of reality can come with real consequences.
If the actor does not act in accordance with his subject's best interests, his actions are unjust.
Thirdly, the actor has to ask himself whether he is indeed as knowledgeable as he thinks, and the subject indeed as ignorant. For example, lets say an actor tries to convince a subject that aliens exist. The actor may believe aliens exist, but he is ultimately ignorant of whether they really do. Similarly, the subject may believe aliens don't exist, but he is just as ignorant. If the actor successfully convinces the subject that aliens exist, ignorance has merely been exchanged for more ignorance. The actor simply had nothing to teach the subject from the start.
If the actor is ignorant of his own ignorance, his actions are unjust.
Lastly, on the topic of manipulation I gather what you are suggesting is to lie in order to convince someone. Spreading ignorance can never be just. You may have brought someone to the realm of true opinion, but in a dangerous way that impedes his chances of reaching understanding. To illustrate my point: The subject may believe that the city of New York does not exist. The actor has been to New York, and can tell the subject the way in order to reach New York (knowledge). If the actor convinces the subject that New York exists (true opinion), but has to lie about the way to get there, he is spreading false understanding, and the subject will inevitably get lost trying to find it.
These are my thoughts. I'm curious to hear yours.