I really highly suggest that you PM SilverSage, as she is by-far the most experienced IRN owner/breeder you will ever find, she has many of them, both pets and breeders, that live both indoors and also outside in an aviary that is pretty open..So I'm sure that she will have more honest, true, and specific information about just how "noisy" IRN's are, how loud they typically are (if she can hear her outdoor birds inside of her home, well, there ya go), etc. I'd ask her any and all questions you have about them, as she'll be able to tell you more and you'll learn more from her than any website you've been doing research on...Because from the sounds of it, as Wrench already said, you've gotten some info on IRN's that at best isn't quite accurate, and at worst is just flat-out untrue...
Any and all species of birds can and do make "noise" at any time of the day or night, and each individual bird is going to be different as well...It all depends on the bird, and also has a lot to do with the environment the bird is in...For example, the first thing that stood-out to me that you mentioned was that you're planning on keeping your bird upstairs in a spare room...Now I know you said that the bird will get a lot of attention, but here's the problem...Most-all birds, especially hand-tame birds who are bonded to their people, absolutely hate it when they can hear that someone is home but they can't see them...This is one of the most common causes of constant/chronic screaming in parrots, and is one of the main reasons that it's suggested that you always put the bird's main-cage in the living room, TV room, den, etc., whatever room in the home where people spend most of their time...So without meaning to do so, setting your bird's cage/environment up the way you are planning to may actually make the "noise" situation much, much worse...
Also, I wouldn't ever make assumptions about or shape your ideas about what a particular species of bird/parrot is like based on things you read on the internet, or even in a lot of books that attest to telling you everything about a certain species of parrot...This is why i suggest that you speak to SilverSage, because she not only has owned/bred IRN's for a long, long time, but she's owned MANY IRN's and has seen their different personalities.
As far as "soundproofing" a room goes, it's extremely time-intensive and expensive to actually "soundproof" a room, so it's better to just do everything you can do to block as much of the sound as you can...There aren't a lot of products you can buy for soundproofing that don't either go inside the walls or get hung/attached over the current wall/panels...Any of the foam squares that you can buy to hang up on the walls are not meant to stop the spread of sound, but rather to dampen the sound inside of the room for acoustics...So the best thing you can do is to do what others have already mentioned, like hanging thick blankets, blocking the air-space under the door, etc.