"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 Tène style material culture, and many specialists believe that Ireland had already been using a Celtic language well before any putative arrival in this period.