Yes, you need to post IRN issues in the Asiatic Ringneck Chat section, but I will try to answer some here.
My Male suddenly had breathing problems and died within the hour.
1. If your male was sick and suddenly died, he must have been hiding symptoms, or was exposed to fumes that killed him. Do you use teflon pans or other items in your home? Scented candles? Air fresheners? What kind of cleaning products were used near him? Did you refrigerate his body and take him to an avian vet for a necropsy to see what killed him?
I then introduced another Male placing him next to the hen's cage, hoping to reestablish my pair. The Hen was aggressive towards the new Male at first but within two days everything seemed ok. I therefore decided to try and introduce the new Male to the hen. Although wary at the beginning, they were not aggressive towards each other. But during the night, my hen severely attacked the new male, and who eventually died.
2. IRN hens are the dominant bird in the breed. When introducing a possible mate to a new hen, all care should be taken because as you have found, they will kill other birds. A slow introduction in seperate, side by side cages should be done. When they start displaying for each other, I would use supervised in cage together, or out of cage time. If they continue to get along with NO agressive behaviors for several weeks/months, I would then introduce them into a different cage together. This way there is no established "territory" in the cage, and neither feels like they are being intruded upon. I would place a baby monitor next to the cage for the next few months, that way if one is attacked during the night, you will hear it and be able to intervene before another bird dies.
3. Your hen may never accept another male. I have an IRN hen that is a confirmed killer, she is a pet bird now, and is kept seperate from my other birds. Her out of cage time is by herself, and she is still supervised so she doesn't bite the other birds through the cage bars.
Please visit the Asiatic Ringneck Chat and ask for more advise there. I do not breed IRNs because of the agression issues they have. Also search the internet for IRN websites (I have one in the link at the bottom of my post), there are some tips on these sites as well.