Original: It was a combination of the names Rose Ussher (the daughter of Henry Ussher, the Anglican Archbishop of Armagh) and Edward Trevor (who came from Brynkynallt in Wales).
Simplified: The name 'Rostrevor' first appeared circa 1618 and was named after Rose Ussher (the daughter of Henry Ussher, the Anglican Archbishop of Armagh) by her husband Edward Trevor (who came from Brynkynallt in Wales).