Archeology evidence shows human presence in Ireland about 33,000 years ago.  After the iceage, the Beaker culture arrived followed by the Celtic Hallstat culture about 500 BCE, and the the Celtic La Tene about 300 BCE.  Christianity appeared about 400 C.E. and by 600 C.E.the early Celtic polytheism had been replaced with Roman Catholicism.  Scandinavian settlement in 8th century was limited, just coastal, but this did have a significant cultural effect.  The Normans appeared in 1169 with a partial conquest beginning 800 years of English political and military involvement. However, the Gaelic influence repaced English culture eventually resulting in armed conflict and the separation of the southern part of Ireland as a Republic, dominated by Gaelic Culture and Roman Catholicism.  The English creation of a plantation system under William of Orange, created hatred of the English and periodic outbreaks of rebellion.  The history of Ireland is so complex. the in order to understand, takes considerable study. I highly recommend Nelson LLewelyn’s series of historical novels. This article is an extraction of Wiki information.  These Links are also helpful:  History of Ireland, Celtic Christianity, Irish Rebellion 1798, The Troubles, The Irish Revolution Period early 1900’s, Great Famine aka Potato Famine, Ian Paisley, Irish Rebel Music.