Hallstatt is the largest and widest known of these, and often is referred to as the Hallstatt Empire. Extending to the North of Hallstatt lies Keltoi, Nathoud and Skye. Each of these countries contains remnants of the old Celtic Empire and is ruled in a different fashion based on their own histories.
One of the effects of the fall of empire was that the Celtic gods have been split into two versions. There are druids (with a small d) that are priests of the various gods worshiped by the many believers in the Celtic World and the many believers on both sides of the Great Mountains. These druid priests spread the Celtic faith far and wide in the Western World. Then there are the Druids (capital D) that hold to old values, such as human sacrifice; and get their powers from the pantheon overall rather than an individual goddess or god. These Druids hold sway throughout the Celtic World.
In the Celtic World there are some differences that other characters coming from the Western World will notice. Society here places Druids and Bards in much higher esteem here than in other lands. In most of the villages and towns scattered across the landscape there will be at least one Bard who is the informal historian and entertainer for the community. Druids are the primary source of spiritual power, and warriors make up the bulk of society. Mages tend to be on the outskirts of society in many places and regarded with suspicion.