Colin B.D. Mark gives only radain for both gen.sg. and nom.pl.
But then in Corpas na Gàidhlig DASG (corpus consisting of many literary works in Gaelic from 17th–21st centuries) has only one instance of na radanan and multiple of na radain. And AFB and Dwelly contradict themselves by giving examples of plural na radain (example in AFB, example in Dwelly). Dwelly also has one example of plural radan.
So in general I’d stick to radain since every dictionary seems to confirm it, and remember that radanan might be a rarer alternative.