Would the French Foreign Legion be it, or is there a medieval/ancient equivalent? I ask this as there are so many elements of the series that are based on medieval and ancient history, the idea of a convict army guarding a remote perimeter is not impossible.
As an order of celibate brothers fighting what their people view as barbarians, one could compare them to the Crusader orders, such as the Knights Templar and Hospitaller. Their place as somewhere to send people to get them out of the line of succession (as with Samwell Tarly) is also something that orders of medieval monks were occasionally used for.