What you are describing is the end of a cul-de-sac. The circle would be to turn around and drive back out. The circle would be county owned right of way. Contact your county zoning administrator and your county road department. It doesn't sound like the cul-de-sac/circle is regularly maintained by the county and I assume you have a gravel drive and the road extending out from the cul-de-sac/circle is probably gravel. It also sounds like the prior owner built the drive off the center line of the circle more into your lots. Is this right?
Enforcement is another issue. The county may send letters, etc. but not all have legal staffs to file lots of litigation. Lots of places that even have attorneys still hire out their litigation and many times they don't incur the expense unless it affects a lot of county residents. Regina described a number of possibilities depending upon what you have in your county, I don't know. If you are in a small, rural, poor, less populated county, and being at the end of a cul-de-sac, you may not get much help and have to enforce your rights yourself. On the other hand, large, rich, urban counties with big staffs may jump right on it cause they tend to be more progressive and don't things to get out of hand.
Post back as to the accuracy of my assessment of your description and also where your are at along with type of county and cities, etc. you are nearby. If you are within 3 miles of a city, you may be in their extra-territorial jurisdiction and then you have to check which one has the zoning jurisdiction but even if the city controls the zoning, it would still be a county road issue. As opposed to a zoning regulation issue which deals with land use, it is really more of a subdivision regulation issue and you will have to see whose regulations apply and with which one where your lots and subdivision are platted. Ask to look at the preliminary plat as well as the final plat. The final plats end up at the register of deeds offices but the preliminary plats that have all the details such as roads, etc. will be at the zoning administrator's office.
A nuisance abatement action may be a possibility in addition to the trespass onto county right of way in violation of subdivision regulations and general trespass law.