The shire system was formalized by the Anglo-Saxons (roughly between the 7th and 10th centuries). It wasn't just a line on a map; it was a system of governance.
The system was largely established by the Anglo-Saxons between the 5th and 11th centuries. When the Normans conquered England in 1066, they kept the shires but began calling them "counties" (from the French comté , the domain of a count). However, the two terms became interchangeable. what is a shire in england