Around the 7th century BC, Celts came to Ireland.

An excerpt from the article 39 facts about Ireland

There is no archaeological confirmation of the great Celtic invasion of Ireland. These could have been invasions by small groups or long-term peaceful influxes and cultural influences of the Celts, which led to the assimilation of the local population and the creation of a Celticized society in the sense of sharing a common Celtic culture and language. They divided Ireland into four provinces: Leinster, Munster, Ulster and Connacht. Before their arrival, the basic units of Irish society were the Tuatha, or petty kingdoms. The entire territory was ruled by a monarch called the Great King.