From about 4500 BC , Neolithic settlers arrived introducing cereal cultivars , a housing culture ( similar to those of the same period in Scotland ) and stone monuments . A more advanced agriculture was to develop . At the Céide 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 . Consisting of small divisions separated by dry @-@ stone walls , the fields was farmed for several centuries between 3500 BC and 3000 BC . Wheat and barley was the principal crops imported from the Iberian Peninsula .