Towards the end of the period, large numbers of stone circles were constructed, although these were restricted to two geographical areas in Ulster and Munster. These, too, probably had ceremonial functions. In a period of time beginning around 500BC, the Celtic culture which was expanding in central Europe reached the Britain and Ireland. While probably not an invasion as such, the Celts arrived in sufficient numbers to replace the language and culture of Ireland's Bronze Age residents with that of the Celts. The Celts were an Iron Age people, and used iron in addition to bronze to make weapons, cooking utensils and other items.
Iron takes more technology to refine, but it also much stronger and more durable than bronze.
The Celts in Ireland divided themselves into over a hundred small kingdoms, or tuath s, each of which was ruled by a king. Sometimes a king had the allegiance of a number of other kings and there were a few kings who had a lot of control. These were kings of provinces. Early maps by Mediterranean sailors and geographers have identified the key tribes in Ireland around the time of Christ and these have been matched with historical evidence and folklore. The Celtic culture was based around a system of honour, whereby warriors gained honour by valour in battle.