We tend to view history from the perspective of today. Nowadays Radnorshire is very much a pipsqueak when compared with, say, London, but was that always the case? In medieval times, when London had a population of around 50000, the lands that became Radnorshire probably had around 16000, not much less than today. Look at history from that perspective and this part of East Central Wales was a much bigger player than it is today. Historians sometimes miss that kind of thing.
Something similar happens when we consider Britain in the immediate post-Roman period. This blog is written in English and not Irish, so it's natural to see the Anglo-Saxons - the eventual victors - as being the greatest threat to Roman Britain. If you lived at the time perhaps you would have seen things quite differently, with the Irish and the Picts posing far more of a problem.
Look at a map of Irish placenames in Wales and we find that they are concentrated in West Wales, including Ceredigion, and from there into the strategic heartland of Britain run two great rivers the Wye and the Severn. Guarding the headwaters of these two river routes you find the old Welsh kingdoms of Gwerthrynion (basically the old hundred of Rhayader) and Arwystli. Gwerthrynion itself seems to be named after Vortigern, a likely military leader of Britain in the early 5th-century. So were these little kingdoms originally established as military provinces against the perceived major threat of Irish expansion into the Midlands?
Anyway here's an interesting link which argues that Stanage - that part of Radnorshire on the Teme, east of Knighton, has important Vortigern connections. Perhaps Time Team should dig them up.