This interesting surname is of Irish origin, and is an Anglicization of the Gaelic "O'Riagain", descendant of Riagan, a personal name of uncertain origin, but may be derived from "riodhgach", impulsive, furious. There were two septs of this name, the most important being the O'Riagains of Counties Meath and Dublin, who were of the "Four Tribes of Tara"; they were very active against the Scandinavian invaders, but lost their importance with the arrival of the Normans who drove them away to Leix (County Laois). The other sept were descended from one of Brian Boru's brothers, Donnchadh, these were of the powerful Dalcassian sept which ruled in Counties Clare and Tipperary, and in Leinster; these two septs were not related. The Maurice O'Regan recorded below wrote a history concerning the arrival of the Normans; he was a diplomatic secretary to Dermot MacMurrough, King of Leinster. The modern surname can be found as (O)Regan, (O)Reagan, (O)Regane and (O)Riegaine. Among the recordings in Ireland are the marriage of Thomas O'Regan and Bridget Halpin on June 16th 1828, at St. Michael's, County Limerick, and the baptism of William O'Regan on March 3rd 1866, at Mallow, County Cork. The first recorded spelling of the family name is shown to be that of Maurice O'Regan, which was dated circa 1125, Leinster, Ireland, during the reign of Turlough Mor O'Connor, "High King of Ireland", 1119 - 1156. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.