This ど is the same ど that shows up in the everyday word けど ("but").
This is a very old element of the language that first appears in the Kojiki of 712, all the way back at the beginning of written Japanese. ど attaches to the 已然形【いぜんけい】 or "realis form", in modern Japanese more commonly talked about as the 仮定形【かていけい】 or "hypothetical form", the verb stem ending in -e for many verbs. This ど is used to indicate a shift in sense to introduce a contrary proposition, much like the English terms "but", "however", "although", and the like. This still appears in modern Japanese, but most often in formal or poetic contexts, and commonly with the emphatic inclusive particle も added afterwards.
- ...とは言えども -- formal, stiff, archaic contexts, seldom used in speaking
- ...とは言っても -- informal, regular, everyday contexts, used in speaking
If you can read Japanese, the 日本国語大辞典【にほんこくごだいじてん】 entry at Kotobank has a good description.