The town's name comes from the Irish Inis - "island" - referring to a piece of ground formed by two courses of the River Fergus, called Inis Laoi ("Calf Island") or Inis Cluana Rámhfhada ("island of the long rowing meadow"). The river still runs through the centre of Ennis and is a well-known trout and salmon fishery. Small sailing boats once made their way up from the Shannon and berthed at Woodquay in the centre of town. The area around Parnell Street and Mill Road flooded routinely until the modern flood defence system was built, though parts of the town still went under water in November 2009 during a severe Atlantic storm cycle that overwhelmed the new defences. Harmony Row Bridge, a pedestrian crossing of the Fergus, opened in June 2009 - a few months before the worst floods Ennis had seen in decades.

The history of Ennis is tangled with the O'Briens, descendants of Brian Boru and Kings of Thomond. In the 12th century they left their seat at Limerick and built the Clonroad Fort in 1210 on the banks of the Fergus - a royal residence rather than a defensive stronghold. The Franciscan friary established in 1240 became their patronage project. When County Clare was formally constituted under Elizabeth I in 1586, Ennis was made its administrative centre because of its central location and its O'Brien association. The town received a charter for fairs and markets in 1610, then a corporation with a Provost, Free Burgesses, and Commonalty. By 1801 it was sending representatives to the Westminster Parliament. Through every reorganisation since - the 1840 dissolution of the corporation, the 1898 conversion to urban district, the 2014 Local Government Reform Act dissolving the town council - Ennis has remained the county town of Clare.

Outside the courthouse stands a monument to Daniel O'Connell - "The Liberator" - who won the famous Clare by-elections in 1828 and triggered Catholic Emancipation a year later. Standing in O'Connell Square, the column marks the site where O'Connell, a Catholic who under existing law could not legally take his seat, was elected by Clare voters anyway. The British government's choice was to deny him the seat or change the law that excluded Catholics from Parliament; under threat of civil war they changed the law. A separate monument to Éamon de Valera, founder of Fianna Fáil and ninth President of Ireland, stands outside the courthouse too. Ennis was a Fianna Fáil stronghold for most of the 20th century, though by the 2009 local elections the party held only one of nine seats on the town council.

In September 1997, Ennis was named Ireland's first - and only - Information Age Town in a national competition run by Telecom Éireann. The £15 million prize bought 4,200 home computers, a computer lab for every school, and a computer in every primary school classroom. Elderly residents were given training. The town's businesses were connected via Ireland's first high-speed ISDN infrastructure. Ennis was a test site for VISA Cash, the early chip-and-PIN payment system. The funding ran out in 2000, and the Information Age Town remained a snapshot of late-1990s techno-optimism more than a foundation for what followed. But for three years, a small Irish town held more computers per capita than almost anywhere in Europe. Ennis went on to win the national Tidy Towns competition for large urban centres in 2012, 2013, and 2021.

Ennis is one of the unchallenged centres of Irish traditional music. The Fleadh Nua festival has run here every May since 1974, the second-largest traditional music festival in Ireland after the Fleadh Cheoil na hÉireann itself - which Ennis hosted in 2016 and 2017, the first time the town had taken it since 1977. Glór Theatre is the main concert and events venue in the town centre. Many of Ireland's finest musicians live in or near Ennis: Kieran Hanrahan won the All-Ireland banjo championship at fourteen and went on to become a radio host; Tony MacMahon, the button accordion player, was born here; the singer Maura O'Connell grew up here; Susan O'Neill writes songs out of the same musical landscape. The actress Denise Gough, two-time Olivier award recipient, comes from Ennis. So does Mark O'Halloran, the screenwriter. Cusack Park hosts Clare GAA matches and the town has six adult soccer clubs - among them the wonderfully named Turnpike Rovers, named for a road that no longer exists.

Ennis sits 14 kilometres from Shannon Airport, the regional airport that opened the west of Ireland to transatlantic aviation in the 1940s. Bus services run hourly between the town and the airport, on to Limerick and Cork and Galway and Dublin. The M18 motorway extended south to Limerick in 2007 and north to Gort in 2010. Ennis railway station connects with Limerick and Galway, and through Limerick onto the Dublin and Cork routes. The West Clare Railway - the narrow-gauge line that once ran out to Ennistymon and Milltown Malbay along the coast - closed in 1961, despite new diesel trains arriving in the 1950s. Its lyrics live on in Percy French's song "Are Ye Right There, Michael?" - a comic complaint about late trains and slow service that long outlasted the railway itself.