Arrival of the Celts Though it may be literally correct and has 2 ref no page numbers though to just say The period between the start of the Iron Age and the historic period AD 431 saw the gradual infiltration of small groups of Celtic speaking people into Ireland with no context is misleading An imported upper class is only one explanation for the arrival of Celtic La T ne style material culture and many specialists believe that Ireland had already been using a Celtic language well before any putative arrival in this period