The Augustinians arrived in the fourteenth century, and the friary they built attracted the patronage of the Dillon family - Anglo-Norman lords who developed the surrounding district from the 1200s onward. Charles Dillon is buried in the Friary alongside many of his ancestors; the family connection runs through the church floor and into the soil of the town. Markets came next, and the trade in cattle, oats, pigs and turf brought a steady flow of money to a region that, like most of east Mayo, balanced perpetually between subsistence and emigration. The town and its hinterland are studded with megalithic monuments - the recurring sign in this part of Ireland that people were burying their dead in stone here long before anyone wrote their stories down.

On 1 April 1921, towards the end of the Irish War of Independence, the IRA's East Mayo Brigade was active around Ballyhaunis. Their commanding officer, Sean Corcoran, was killed in a gunfight with British soldiers six kilometres north of the town at Crossard crossroads. A cross marks the spot. The same day, the IRA killed a member of the Black and Tans in the town centre, and the Black and Tans retaliated by seizing Michael Coen, a local IRA volunteer who appears not to have taken part in the day's fighting at all. They executed him. A monument to Coen stands on the Cloonfad to Galway road. In May, Patrick Boland, captain of the Crossard company, was killed by Crown forces too. The Holywell Ambush, which gives its name to one of the local memorials, was part of the same months of attack and reprisal. A high cross now marks where the ambush happened.

Ballyhaunis is home to Ireland's first purpose-built mosque - the first mosque in the country outside Dublin. It grew out of the Muslim community that took root around a local meat-processing plant supplying halal product to Middle Eastern markets. The town's demographic profile has shifted steadily ever since. By the 2016 census, Polish and Pakistani residents made up the largest immigrant groups, with people from many other countries settling alongside them, giving the town the highest proportion of foreign-born residents of anywhere in Ireland. By the 2022 census, the population had climbed to 2,773, with the foreign-born share still well above the national average. Ballyhaunis became one of the most-studied examples in Ireland of how a small rural town absorbs and reshapes itself around immigration. The Friday call to prayer now carries across a square laid out around the medieval Augustinian friary.

Small towns sometimes get chosen for things that other towns do not. In 1999, Ballyhaunis was one of several European Union sites selected to trial a local currency, the ROMA, designed to stimulate local trade and ease residents into the new euro. Annagh Magazine, founded in 1977 by the local Junior Chamber and named after the parish, has appeared every December since Christmas 1978 - a remarkable run for a community publication. MidWest Radio, based in the town, broadcasts to Counties Mayo, Galway, Roscommon, Sligo and Leitrim. The Dublin-Westport line passes through, with Ballyhaunis railway station opening on 1 October 1861 and still in regular service. The 150th anniversary of the station was celebrated by a dedicated edition of Annagh Magazine in 2011. Bus Eireann Expressway route 64 stops in town on its long run from Galway to Derry.

Bill Naughton was a playwright with Ballyhaunis roots whose 1963 play Alfie, about a feckless, philandering London cabbie, was adapted into two films of the same name - the 1966 original starring Michael Caine, and a 2004 remake starring Jude Law. Pamela Uba won the Miss Ireland title in 2021 and called Ballyhaunis home. Sean Flanagan played Gaelic football for Mayo and then went into government. Anthony Jordan writes biographies. Jim Higgins served as a senator, a TD and a Member of the European Parliament. Keith Higgins played full-back on the senior Mayo football team. The town that grew up around a 14th-century friary keeps adding new names to the Wikipedia entry, in patterns no medieval mapmaker would have predicted.