- Synod of Whitby decrees Roman supremacy over Irish Church in conflict over correct date of Easter.
- Cult of St. Patrick established through hagiographic "Lives" (mostly fictional biographies).
- Start of Viking raids on Irish monastic sites (sometimes attracted by the prominent round towers) and settlements.
- Vikings start exploring Irish mainland.
- Vikings use the Boyne, the Liffey, Dublin Bay and even Lough Neagh to overwinter.
- Vikings and Irish establish first diplomatic relations and forge alliances.
- With the sack of Limerick Irish tribes begin resistance against expanding Viking rule.
- Brian Boru becomes King of Munster.
- Brian Boru defeats his main Irish opponents and accepts the surrender of the Viking King of Dublin, Sitric Silkenbeard.
- Brian Boru becomes High King of Ireland.
- Brian Boru defeats the Vikings in the Battle of Clontarf ... but is killed himself.
- With the establishment of the first Cistercian house at Mellifont a new form of monasticism enters Ireland.
- Pope Adrian IV (an Englishman) grants possession of Ireland to Henry II of England (but even the papal bull Laudabiliter may be an English forgery).
- King of Leinster Mac Murchada (Dermot McMurrough) emerges as the loser in inner-Irish power struggles, flees to England and starts to raise a mercenary army to conquer Ireland.
