The Celtic people occupied great stretches of land across central Europe by 500 BC. They first arrived in Ireland around 300 BC, subsequently controlling and influencing this land that the powerful Roman culture never reached.
The Pope in Rome (Celestinus the First) sent the first Christian missionaries to Ireland in the 5th century, including St. Patrick. The Catholic faith spread rapidly, and by the turn of the century abbeys and monasteries were beacons of hope across Ireland.
This somewhat peaceful Celtic world was first invaded by the Vikings in the late 8th century. Their overpowering raids of monasteries and villages continued through the 10th century, and they eventually built permanent power base settlements at Cork, Dublin and Waterford.
Localized fighting over land and property continued for decades, but over time, some Celtic family groups and Vikings reached alliances - sort of a workable, live together understanding. That ended when the Irish King Brian Boru defeated the Danish Vikings in 1014.
In 1169, aggressive English and Norman forces crossed the Irish Sea; Dublin (a Viking stronghold) and other cities quickly fell, and the all-powerful Pope (in Rome) declared that Henry II (the English King) was now the "Lord of Ireland." As you can imagine, that didn't sit very well with the locals.
Centuries of conflicts and localized wars continued, and finally, the embattled Irish chiefs were forced to retreat into the northern province of Ulster (parts of which are now Northern Ireland). By 1607, most, if not all, would flee this land and the English saw an opening.
They quickly instigated a policy called 'plantation,' where waves of Scottish and English Protestants literally took the available land from the now weakened Catholics, laying the groundwork for the strong differences and bitterness that remain to this day.
Understandably, the now angry and proud Celts rebelled, and bloody riots soon followed. Then, after the native Irish supported the losing side in England's Civil War, the worst for Ireland was yet to come.
In 1649, Oliver Cromwell, England's "Protestant" Lord Protector, (on the winning side in that war) led a punitive expedition into Ireland. The massacre was bloody, brutal and most destructive - and not yet forgotten.
As the English enforced strict (none of this, none of that) new laws on the Catholics, Protestant powers increased, and Ireland and its northern areas grew further apart, primarily for economic reasons.