Juncus arcticus ssp. arcticus; The Arctic Rush has two subspecies ("arctica" and "intermedius"). Also, there is a similar species being Juncus baltica "Baltic Rush"). The arctica subspecies has very compact inflorescences whereas the Baltic Rush has very loose inflorescences. You probably have guessed by now: the "intermedius" subspecies of Juncus arcticus holds the middle. Another difference is that the Arctic rush has darker black inflorescences. See further description of J. balticus and J.arcticus ssp. intermedius at their specific pages.
The Arctic Rush can be identified by the rather loose culms and the inflorescence about 3/4 of the leaf. (the part on top of the inflorescence is technically speaking actually a bract). It grows all over Iceland especially where wet and drier soils meet.
It is a member of the rush family (Juncaceae).