In 1166 Dermot MacMurrough was forced to flee his kingdom after being defeated by Rury O'Connor, High King of Ireland. Arriving in England, he sought the help of King Henry II, who allowed him to recruit an armed force. MacMurrough returned to Ireland in 1167 with a small force and won back some of his ancestral lands. He was followed in 1169 by the Anglo-Norman force that restored him to his kingdom.
Reference: National Library of Ireland Expugnatio Hibernica ('The Irish Conquest') by Gerald of Wales (date: completed 1189) By permission of the Council of Trustees of the National Library of Ireland.