Unless Cha-cha is physically in the cage with the cockatiels at all times that he's not with me, he screams until I cover him up to make him settle down. But it's not fair to him to keep his cage covered. What I have ended up doing is only covering the side where he can see me and allowing him to still be able to see the other birds and out of the windows of the house. I would house him in the same cage because it's large enough but I'm afraid one of the smaller birds will get hurt and that wouldn't be fair to put them in that kind of predicament. I've discusses with my husband, how to separate a part of the cage for him but we've not come up with a suitable solution for that yet.
I have tried walking out of the room when he screams badly but that only makes him call out louder and more incessantly. Good thing that I don't live in an apartment building because I would've been evicted by now!
Even if he is in the same room with me, unless he's got my undivided attention he will just scream incessantly. I cannot allow Cha-cha to sit on my shoulder because he also likes to bite for a reaction which is another problem altogether that I won't address here: but that seems to be the only solution that makes him happy when he's with me.
Whenever he's behaving as I would like I always go over to his cage, give him a treat and/or talk to him but as soon as I turn to walk away he makes the most obnoxious and ugly sound and bites at the bars of his cage or lunges in my direction.
I have even resorted to playing a training CD to teach Cha-cha how to say a new word or two, since he doesn't seem to be learning from me, which keeps him quiet while it's on but as soon as it's turned off he just starts screaming again.
I've tried to engage him in some games I've learned from other folks I know that have birds but he doesn't seem too interested in them.
I've had Cha-cha for at about 5 years and I love him but I'm coming to the conclusion that this problem is WAY more than I seem to be able to handle and I cannot afford to take him to a behaviour expert. Also, I'd like to say that I do realize that some of the vocalizations parrots make are very loud (fine, goes with the territory) but I don't believe that every single sound they produce is intended to be at maximum volume. I'm getting to my wits end. Perhaps he would be better off with a breeder or with a mate since he seems to be the kind of bird who needs constant attention. For now I've been letting him spend as much time as he pleases with my cockatiels during the day so he is good but as soon as he goes back to his own cage which is right next to theirs he's back to screaming unless he is covered up for the night. The only exception to this rule is when I give my birds a shower, he quiet for a while afterwards while he preens and dries himself but otherwise he just starts again. Don't get me wrong, Cha-cha's come a long way since I first got him, at least now he has some quiet times which he didn't have at all when I first brought him home, but for being as intelligent as parrots are he sure is taking his sweet ol' time getting the point!
Is there anything that I'm doing wrong or anything else I could try?
Tzigane
As soon as ChaCha can seem me or my husband in a room close by he will scream to try to get our attention. We've been leaving the room as instructed but I've only noticed a very slight tapering off of the habit; not really what I would consider an improvement.
Also I think something was mentioned about training, if your bird is tame enough I don't think it's ever too late. Some training will help teach him some much needed dicipline, which I'm positive would also hlep to improve his screaming. Dicipline, something I need to teach more of to my little birdie! He keeps jumping on my keyboard as I'm typing this post, that he already erased once!! 
You asked if you can't change the behavior how can you make it easier on everyone. I think that if by covering his cage he quiets down then there is no harm in keeping it covered for him. Or by keeping him with other birds, spending time with him whatever you are willing to do to keep him queit and happy! The amount of work you put into it is up to you. My friend also has an IRN, when he (the IRN) is ready to go to sleep he will call to them to cover his cage. He also does this if his cage is not clean! He is telling them to come do it otherwise he lets them know he is not happy about it. If these things are done however he is queit as can be!So, even if you feel it's unfair to Cha-Cha to cover his cage so often, if it's what makes him feel safe and secure then no harm done! Obviously no change in his behavior would be the worst case scenario, but I'm sure we'll find something that helps hopefully. 
To date Ricki is the only cockatiel that has ventured over to ChaCha's cage via the bridge that I put up between the cages, but then again Ricki is afraid of nothing!
he is good as gold. As a matter of fact, when he's with the cockatiels he eats with them, follows them around, talks to them, whistles to them and sings to them. He's happy as a clam in a mud bed. I know that I cannot house the cockatiels and ChaCha together in the same cage. What do I do now?!