It seems that the Pirmins listed in the
German Wikipedia are all from Switzerland (except one who live at the Swiss frontier). And unfortunately they don't give its etymology...
The Durs-Dorian equivalence seems folk etymology. The Duden Vornamenlexikon says that it is of uncertain origin, maybe from Old High
German ''thurs/turs'', demon/giant, or from ''Sankt Urs''>''Sank Turs''>''Durs''. So it probably is a regional southern Germanic name and not an actual exclusive Romansch name (it's just diffuse among Romansch people).