[JeffF.]

I don't think Gandalf's ring necessarily provided protection for the Shire. THe Shire was relatively small and the population was small, Eriador alone was an immense area (the distance from the Shire to Morder being equivalent to that between England and Bulgaria) and by the time of the War of the Ring there were few servants Sauron could call upon in these lands.

The most protection seems to have been provided by the Rangers, Dunedain of the North, who Gandalf asked to keep watch on the Shire and whose efforts appear to be the main reason why Sauron's servants could not find it. Sauron also had incomplete information as Unfinished Tales, the Search for the Ring shows, he thought the Shire might be in the Vales of Anduin and possibly withing Galadriel's realm.