No, medieval Iceland was not anarchist. It was a collection of chiefdoms with a shared legal tradition, a central legislative assembly, and a judicial system.

Chieftains were not territorial lords, and didn't have much coercive authority. The exact origin of Icelandic chieftainship is a little unclear but the office apparently "evolved" out of the pre-Christian priestly office. Free farmers were not required to accept the authority of any individual chieftain, and sometimes transferred their loyalty from one to another if circumstances warranted. The office of chieftain was itself treated effectively as property. It could be inherited, but also purchased, received as a gift, or even shared. Membership in a particular family or clan was not a necessary requirement, at least not initially.

Chieftains met annually at an assembly called the lögrétta, which confirmed old laws, amended existing laws as necessary, and adopted new laws. The highest official, the "law speaker" was elected to a three-year term at the lögrétta as well. The law-speakers main, and possibly only, job was to recite one-third of the entire body of Icelandic law each year during the lögrétta.

Another annual national assembly, the Althing, held law courts for the various regions ("quarters") of the island. Regional assemblies and courts were also held in the quarters. Judges didn't have special training, but were selected by the chieftains from among the free adult men and heard cases as part of a whole panel of judges. Thus, they were almost more like the modern jury.

Iceland was also officially Christian from A.D. 1000 (which actually happened through legal process at the Althing), so there was also Church authority in Iceland. Interestingly, another responsibility chieftains often assumed was building and staffing Christian churches. In some cases, the chieftains arranged for priests to serve these churches, while in other cases, chieftains themselves became Christian priests. Iceland had Christian bishops from the 1050s onward.

Source: Jesse L. Byock, Medieval Iceland, University of California Press, 1988.

What do you mean by Anarchist?