Pretty much what it says on the tin. Is there an RPG system that has been licensed for the Redwall setting created by Brian Jacques? I expect the answer to be "No" so as a follow up, has there been any material created to re-skin an existing system to take place in that setting?
You want Mouse Guard.
There's no licensed game for Redwall, but the Mouse Guard RPG is the closest you'll get. Based on the comic book by David Petersen, in the world of Mouse Guard, "mice struggle to live safely and prosper amongst harsh conditions and a host of predators" — which it has in common with Redwall. David Petersen himself commented upon Redwall with respect to Mouse Guard in his blog post memorializing Brian Jacques:
Redwall was as much an influence on Mouse Guard as it was an anti-influence. It was while I was developing the ideas for my own group of mouse rangers I was handed the first book in the series. I thoroughly enjoyed Brian's writing, and his gift for including vocal accents into the text. It charged in me even more desire to pursue my love of animal stories and medieval cultures.
However, I knew instantly that I could never top what Mr. Jacques was doing. So when I say his books were a counter influence, I only mean that I had to set out in a different path from what he was already master of.
The Mouse Guard RPG deals with fantasy rodent and smallfolk culture, handles combat well, and is a beautiful game besides.