Welcome to the community! I'm sorry these issues have popped-up, but as SilverSage mentioned, it may very well be hormonal...Or it could also be a territoriality thing...
***By the way, at around 4 years-old, if your IRN hasn't gotten it's ring yet, then she is a female for sure, not a he...So if she is hormonal right now, you need to make sure she has no access to anywhere small and dark, such as any type of boxes/nest-boxes, "Huts" like the "Snuggle Huts", tents, etc. in her cage, or that she can't get under furniture, or that she has nothing in her cage that could be seen as a "nest" or "nesting material", such as straw, bedding, wood chips, towels, blankets, or any pieces of fabric, as all of this will increase her hormonal behavior and could very well lead to her laying infertile eggs, which is not good for her health...I'd make sure she also has constant access to a Cuddlebone and a Mineral Block both inside of her cage, so that if she does happen to start laying infertile eggs, her Calcium and Phosphorous levels will be high enough that her body won't start leeching it from her bones...And they also tend to keep their beaks filed/non-pointed as well...
****I totally understand how you feel about "isolating" your IRN, however your very first priority HAD TO BE keeping your Sun Conure safe, as you're correct that your IRN could very easily injure her very badly or kill her. This is a situation that commonly results in lost toes, lost eyes, lost beaks (this happens all the time and it's horrible because usually too much of the beak is lost to grow back, and they have a permanent hole in their faces), etc. And honestly your Sun Conure's beak is also large enough that he/she could also badly injure the IRN; even though the IRN is the "aggressor" in this situation, even the most snuggly, cuddly, tame bird will become extremely aggressive and violent when being attacked, and it's usually a situation that results in BOTH BIRDS being seriously injured or killed. So as SilverSage already suggested, what you need to do is put the IRN back in her cage when you settle-in on the couch for "snuggle-time"
with the Sun Conure, at least for right now, to protect them BOTH from each other. That being said, you want to make EQUAL out-of-cage-time for both birds, so if your routine is to snuggle/cuddle with your Sun Conure at night before their bedtime, then that's fine, keep doing that, but during the time before snuggle-time with your Sun Conure, you should put your Sun Conure in his/her cage and let your IRN out for equal out-of-cage time, and then switch them out...And if it's already dark out when you have snuggle-time on the couch with your Sun Conure, then I'd probably start covering your IRN's cage with a sheet/blanket right after you put her back in her cage and just before you get your Sun Conure out of her cage. It's bedtime for her anyway, or it will be soon if it's dark out, and that way she's not going to be able to "watch" you cuddling with the Sun Conure, instead she'll just go to sleep; even if you have the TV on it won't bother her or keep her from sleeping if you cover her cage...
****I'm going to give you my personal and professional experience/advice/education of clipping a bird's wings, and I'll most-likely get negative responses to it because it's a very controversial issue, and people feel strongly about it. However, what people need to keep in-mind but they never do and rather they just start yelling and arguing is that clipping your bird's wings is a personal decision that every individual bird owner must make for themselves, their particular situation, their particular household, and their family. This is not a decision that anyone can or should make based on the beliefs/principles/ethics, whatever you want to call it, of other people...No one lives the same lives, no one has the same household situations, no one has the same birds with the same personalities or behaviors...And honestly, when it gets down to the point that a bird's behavior is potentially dangerous, then it's sometimes actually irresponsible to not clip your bird's wings. So above all else, regardless of what anyone else tells you, including me, YOU and the others in your household are the ones that ultimately have to make the right decision for yourselves and your birds.
****Now please don't get me wrong, all of my own birds are fully-flighted, none of them are clipped...And I don't at all believe in keeping your bird's wings clipped for their entire lives/indefinitely, nor do I believe in just clipping your bird's wings simply because you think it creates an "easier" situation for you. That's not fair to your bird(s)...However, other aspects of wing-clipping that people forget or just ignore due to their anger are that #1) It's completely temporary, and if done correctly and responsibly their wing-feathers will grow back within 2 months or so, #2) It causes them no pain at all, it's no more stressful for them than getting their toenails clipped, and much less stressful for them than having their beaks trimmed/filed, and #3) Depending on the situation, having your bird's wings clipped can actually IMPROVE a bird's life greatly, and potentially for the long-term! And I think that in your particular situation, since your IRN was most-likely not hand-raised and is not hand-tame at all, that having her wings clipped temporarily would not only keep you and your Sun Conure safe from "air-attacks" by your IRN, but if you make the most of the time her wings are clipped, it could also serve to hand-tame her and make your relationship with her much stronger/friendlier/directly interactive, and make the need for caging either bird totally unnecessary....So having your IRN's wings clipped temporarily could actually improve your situation all the way around.
I have hand-tamed many, many parent-raised/non-tame Budgies and Cockatiels throughout my life, along with other species of parrots at the Avian/Reptile Rescue that I've worked at for over 8 years now. And even though clipping a surrendered bird's wings is not my decision at the Rescue, as I'm not one of the people at the Rescue who works on their training/behavioral issues, I have still used their wings being clipped to my advantage...
First important rule to wing-clipping is that you want to have it done by someone who knows what they are doing, like a Certified Avian Vet/Avian Specialist Vet, or an experienced breeder, or possibly the owner/employee of a Bird Shop (not from Petco/Petsmart or some random pet shop that doesn't specialize in birds), AND regardless of where you take your bird to have her wings clipped, even if it is a Certified Avian Vet, you must always be sure to DEMAND that they only clip the outermost 5-6 Primary Flight-Feathers ONLY, no more and never into the Secondaries,
and that they always clip BOTH WINGS equally, never just one wing. This will ensure that she will still be able to glide easily to the floor and have complete control and that she won't drop to the floor like a stone. She just won't be able to get any altitude; also, you always must get both wings equally clipped and not just one, as some people still insist on doing, because if you only clip one wing they have no control at all, they will have no balance at all and they fall off of things, glide into things, and it also puts very huge stress on their spine and causes permanent spinal damage. So you have to be certain that you request/demand that they clip your bird the way you tell them to, and if they refuse or try to tell you that you should clip "more" than just the outermost 5-6 Primary Flight-Feathers or that you should only clip one wing, then you get your bird and get out of there, and find someone else to do it! Period.
