 From about <unk> BC , Neolithic settlers arrived introducing cereal <unk> , a housing culture ( similar to those of the same period in Scotland ) and stone monuments . A more advanced agriculture was to develop . At the <unk> Fields , preserved beneath a blanket of peat in present @-@ day County Mayo , is an extensive field system , arguably the oldest in the world , dating from not long after this period . <unk> of small divisions separated by dry @-@ stone walls , the fields were farmed for several centuries between <unk> BC and 3000 BC . <unk> and barley were the principal crops imported from the Iberian Peninsula . 
