In the Meath/Dublin region it was borne by one of the Four Tribes of Tara, prominent in the wars with the Vikings. After the Norman invasion they were dispossessed, and migrated west wards to the area now part of Co. Laois, where their descendants are still to be found.
A second family claims descent from Riagan, a nephew of the eleventh-century High King Brian Boru; their homeland was the historic kingdom of Thomond, in an area now covered by parts of Co. Limerick.
East Cork, around the modern town of Fermoy, was the original territory of the third family of O Riagain. Their influence in the wider area of east Cork is recorded in the townland names of Coolyregan in Brigown parish, and two Ballyregans, in the parishes of Cloyne and Carrigtohill. Like many other native families, however, they were driven out by the Normans and their allies and by the sixteenth century, most members of this family had migrated to the south-west, and it is with west Cork that the name is most strongly linked today. The main seat of the family was at Ballinaclogh, in the civil parish of Ross.
There would also appear to have been one more family of the name, based in Connacht, where the surname is today still numerous in counties Mayo and Roscommon.
.Sir Teige Mac Shane O Regan (1629 - 1698?) was the last chief of the name of the West Cork family. He was an officer in the Jacobite army in 1690 and for his part in the defence of Charlemont fort in Co. Armagh was knighted by King James. After the battle of the Boyne he became governor of Sligo and in 1691 left for the Continent with the rest of the defeated army. The O?Regan lands at Ballinaclogh remained in the family until the early 20th century.
Ronald Reagan (1911 - ), 40th President of the U.S., was also of the Cork O?Regans, though his ancestors were among those who were not displaced from east to west Cork. Before becoming President, he had a long career as a broadcaster and film actor and was Governor of California from 1967 to 1974.