The name Ennis comes from the Irish word “Inis”, meaning “island”. This name relates to an island formed between two courses of the River Fergus on which the Franciscan Friary was built. The past of Ennis is closely associated with the O’Brien family, who were descendants of Brian Boru. In the 12th century, the O’Briens, who were Kings of Thomond, left their seat of power in Limerick and built a royal residence at Clonroad on what was then an island. In 1240, King Donnchadh O’Brien ordered the construction of an extensive church which he later donated to the Franciscans. The centuries which followed bore great activity. The Friary was expanded and students came in great flocks to study at the theological college. The Friars, who were free to move about, met the spiritual needs of the local population. It was a religious centre until the Dissolution of the Monasteries.
Because it never had town walls, Ennis became a location for merchants from Limerick when Catholics were forbidden to reside in the walled towns by the Penal Laws, and much of its past prosperity is attributable to this influx. It became a thriving market town in the late 18th century and this expansion continued unabated throughout the 19th century, except the period after the Famine c. 1850.
in the colonial period, a number of landmark structures were constructed, including the Mill and Courthouse. The town contains a number of old military barracks, most notably the Old Military Barracks on the Kilrush road. Many locals served in the British Army in the First World War. The Clare Road and Clonroad areas contain terraced cottages built in the early 20th century to house soldiers. On Station Road, then called Jail Road, a gaol once stood.