It is not too late to train the dog.
I do not know whether you will ever be able to trust the dog completely loose around chickens, but you can definitely make things better than they are now.
Just training the dog to obey you can help quite a lot. For example, if you tell her to sit and she sits, she cannot be chasing chickens while sitting!
I have seen almost exactly this story a few times before-- tieing the dead chicken to the dog does not teach the dog to leave chickens alone, but it does let the dog eat more of this chicken.
If you do your best to keep Rosie out of the chicken yard, does it work?
If not, I suggest that you improve the fence of the chicken yard.
Also, consider putting a leash on the dog and tieing the leash to a sturdy belt around your waist. (Or run the belt through the handle loop of the leash, before you put the belt on.)
That lets the dog spend time outdoors with you, prevents her from running and chasing chickens, and lets you grab either the dog or the leash at any time if she misbehaves.
I think you really need to improve the chicken yard and maybe the pen inside it as well. If your own dog can get in, so can a stray dog, or a coyote, or probably a fox or raccoon.
So start by keep her on a leash, or with a secure fence between her and the chickens, or inside the house, or any other arrangements that make it completely impossible for her to get to the chickens. That will keep her from doing it again while you work on training.
Also, make her practice obedience (heel, sit, come, and so forth) on a leash, in a place where she can see and smell chickens but not reach them (like outside the chicken yard, AND on a leash.) This makes her obey YOU, even when the exciting chickens are right there. It does not directly teach her that chicken-killing is wrong, but it does start to teach her that she CAN control herself even when sort-of near the chickens.
If she gets really good at obeying on leash, outside the fence, you could start to work her off-leash outside the fence, and on-leash inside the chicken yard.
I'm guessing you can train her to leave the chickens alone while you are present and paying attention, but it might take months or even a year or more of steady work to reach that point. Whether you can ever trust her unsupervised, I have no idea, but certainly not in the next few months.
I have also heard mixed results, and so I would probably try basic training methods first (leash and normal collar.)
I might consider an invisible fence (shock collar that activates when the dog goes near a certain boundary), and put that right along the chicken yard fence. The shock might be enough to keep her from destroying the physical fence, and the physical fence will keep her from running right through the area that shocks her (my former neighbor had a dog that ran right through his invisible fence all the time-- he had learned that once he went far enough, the shock stopped.)