Complex sentence: It was named after the historic kingdom and province of Mide (meaning "middle"), and is today also known by the nickname "The Royal County" due to its history as seat of the ancient High Kings of Ireland.
Split into simpler sentences: It was named after the ancient Kingdom of Mide (meaning "middle"). It is colloquially known by the nickname "The Royal County" due to its history as the seat of the High King of Ireland.