From an Irish surname, an Anglicized form of Ó Riagáin, derived from the given name Riagán. This surname was borne by American actor and president Ronald Reagan (1911-2004).
As a given name, it took off in popularity during the 1990s. It has been more common for girls in the United States probably because of its similarity to other names such as Megan, Morgan and Regan.