Never had a problem with two male IRN's only actually with two females have I had issues (same with a friend). Males will tend to slot into a dominance and submissive role pretty easily.
Regarding friends, IRN males are very quite flexible, hens are and can be just as flexible, with good parenting.
If you were going to get another IRN I would honestly suggest a cock bird, if your bird is used to larger birds, then you can look at any of the asiatics including alex's etc, else stick to smaller sized birds like the smaller conures or smaller asiatics like moustache, plum headed, slaty headed etc... all make good aviary companions with IRN's in experience. When doing this however, try to stick to the males... simply due to the fact some of these guys can breed together, and will still give it a shot given the chance.
If you were after something with a bit more of a cuddle factor, a greencheeck conure would be a good idea, in this case either male or female wouldnt be a worry at all. Regarding conures... they can 'sometimes' overwhelm an IRN... they have no problem pushing past the largest of birds to get attention. If you wanted to have a safe bet, any of the conures around the greencheek size are generally less dominant over birds like IRN's.
Princess parrots can be good IRN friends (had a blue princess with my lutino IRN for ages). They make lovely chirpy friends, promoting a softer call from your IRN as well, both my IRN and my princess shared songs...
Really, spend the time looking at your budget, and when you have an idea on what you want to spend, take a look at the options from above (or any other ideas someone else might have) and see which you find... always look for a young bird that comes to you and enjoys being around people more so than others.
With the addition of another bird, you will have to remember, they get jealous... so suddenly attention / toys etc need equal share
Else you can see some weird and unhappy reactions from your birds.
Regarding tiels... i have a friend in NSW now who had pet tiels with pet IRN's and a pet sun conure for ages... never had any problems.
With anything to do with birds... it takes time, supervision, and a touch of luck.
is doing well and become a real fun bird. He is about 3 and a half now. We want to add another bird to the flock and had some questions. Syd LOVES other birds and wants nothing more than to spend time hanging out with them. He craves it so much from my tiel (we dont let him near her due to the size issue) he sits near her cage and says over and over- Im okay 