Bagenalstown (/ˈbæɡnəlztaʊn/ BAG-nəlz-town), officially named Muine Bheag (Irish: [ˌmˠɪnʲə ˈvʲaɡ]), is a small town on the River Barrow in County Carlow, Ireland.

In the eighth century, Saint Lappan reputedly founded a monastic school outside Bagenalstown in the townland of Donore. He is the patron saint of Bagenalstown.

The town grew within the townland of Moneybeg (Irish: Muine Bheag, "small thicket"). In the 18th century there was a small hamlet there. Walter Bagenal decided to build a town on the site, to be named "New Versailles" and modelled after Versailles in France. However, shortly after building began, the coach route from Dublin, which had passed the location, was changed so it crossed the River Barrow a few kilometres away, at Leighlinbridge, instead. Bagenal abandoned his plans, having built only a courthouse. It was not until the arrival of the railway in 1846 that the settlement began to grow into a town.

In 1911 the town became the first in Ireland to install dual-language street signs, which remain in place today. Following the creation of the Irish Free State in 1922, the town commissioners chose to drop the English name "Bagenalstown" in favour of the Irish Muine Bheag. Several other places in Ireland were renamed around this time. The decision proved an ineffective one, and "Bagenalstown" remained in common use among locals. In a 1975 plebiscite, 77% of residents who voted opted to change the name back, but the proposal failed due to insufficient turnout.

The town's coat of arms, granted in 1999, shows a lattice of blackthorn branches on a gold field, above a field of ermine. The gold and ermine reference the arms of the Bagenal family (as well as the arms of County Carlow), while the blackthorn lattice references the town's Irish name. A wavy fess bisects the shield, representing the River Barrow. The motto is uimhir gan choisc or "the irrepressible number". This is an ancient name for the number nine, referring to its unique mathematical properties (any multiple of nine will have nine as its digital root), and was chosen because the town commission has nine members.

Bagenalstown's history is strongly associated with the milling industry. Rudkin's Mill marked the start of this industry. It was erected by Henry Rudkin between the end of the 17th century and the start of the 18th century. Shortly after the opening of Rudkin's Mill another mill was opened by Owen Murphy; this would be more commonly known as Lodge Mills.

In 1792 the Barrow Navigation Company began their plan to link the town and river to the Grand Canal. The construction of the canal allowed boats of up to 40 tonnes to transport goods directly to Dublin. In 1845, record numbers were recorded as 111,225 passengers and 286,601 tonnes of goods were transported along this route. Many of the boats that were used for transporting goods on this canal were owned by corn buyers and maltsters. This allowed them to efficiently transport the goods they needed direct from Rudkin's Mill and Lodge Mill.

Bagenalstown very quickly transitioned into a highly prosperous town, and became recognised as a post, boat, mill and marketplace hub. The tradition of milling was also joined by salt and gas works in the 18th and 19th centuries. The canal route very quickly declined once the railway had been introduced and in 1852, only a small number of trading boats remained on this river. As of 2018, the canal was only used as a tourist attraction.