I'm telling you, you don't need to translate "one". In Old Irish you'd either say in deorad, "the exile", or in deoradach, "the exiled (one)". In Modern Irish, an deoraí would do just fine. If you absolutely want to use the same construction as in English, you'd say something like an ceann ionnarbtha, I think, but that would be very clumsy and sound even less like a plausible nickname (emphasis on nickname; it is NOT a name in any case).