Romulan Institute Computer Interface
Vulcans were once an extremely violent and emotional people, waging almost constant warfare upon one another. (TOS: "Balance of Terror"; ENT: "The Forge") As their technology improved, the Vulcans eventually reached a point where their violent nature threatened the extinction of their species. (ENT: "Awakening"; TNG: "Gambit, Part II") Around 370 AD, the philosopher Surak led his people in a great reformation to embrace logic and pacifism called the Time of Awakening. One group, known as "those who march beneath the Raptor's wings", rejected Surak's teachings and fought against his followers. (TOS: "The Savage Curtain"; TNG: "Gambit, Part II"; ENT: "Awakening")
At the end, the dissidents lost the terrible atomic war that ensued and left their homeworld. During their travels, some split away to form the Debrune civilization, which became extinct long before the 24th century. Eventually the group settled on two planets named Romulus and Remus, and thus laid the foundation of the Romulan civilization. (TNG: "Unification I", "Gambit, Part I")
In several non-canon Pocket Books novels, Romulus and Romulan are actually the appellations given to them and their world by the Federation; the Romulans refer to their own race as the Rihannsu. Some of these books, predominantly , discuss the Romulan migration as being led by a Vulcan named S'Task, a follower of Surak who turned away from Surak's teachings and left Vulcan in the 3rd century.
The above taken from Memory-Alpha