The two phenomenon happened, at least for the greeks, but as the roman assimilated part of the greek geographic knowledge, it can be applied to the romans too. The most common situation would be to derive a country from the name of the tribe inhabiting it. Roman gaul was structured in provinces and cities. Those cities and their territory corresponding to one tribe. One of the best exemple would be Trier in roman germania. The roman name of the city was Augusta Treverorum meaning Augusta (city) of the Treveri (people). You can clearly sees how the romans saw bigger groups of populations: the gauls, the germans and individual tribes and used them to name places. Now there are also names wich describe peoples like the ichtyophages: the fish-eaters but those are adaptation of greek names and used to designate remote regions wich are outside of their reach or in mountainous, difficult to reach areas.