In the sixth century, Saint Abban of New Ross - a contemporary of Patrick - founded a monastery at a place called Rosglas, on a curve of the Barrow. He put it in the charge of his protégé, a young monk named Evin. The name comes from the Irish word eimh, meaning swift or active; Latinized as Eminus. Evin had political instincts. He brought monks with him from his native Munster, which earned the place the local name Rosglos-na-Moinneach, the greenwood of the Munstermen. More importantly, he negotiated special status for his foundation: the area around Monasterevin was placed outside common law, made a sanctuary, given protection that the local kings respected. His personal bell became famous as a swearing-implement; tribes from across the region came to take oaths on it, knowing those oaths would hold. He co-wrote the Tripartite Life of St. Patrick. His political writing, the Cain Emhin, survives. He was, as the Wikipedia article phrases it, a sixth-century spin doctor.

Saint Evin's monastery faded around the time of the Viking raids. A Cistercian abbey took over in the twelfth century, under the patronage of Dermot O'Dempsey. The abbots of Monasterevin held seats in the Irish Parliament while quietly assisting rebels against the English crown - a balancing act that would have made Evin proud. Henry VIII suppressed the abbey in 1541. King James I granted the lands in 1613 to Sir Adam Loftus. By 1699 the estate had passed through marriage to the Moore family - the Earls of Drogheda - whose fourth Earl sold his Mellifont estate and made Monasterevin the family seat. The Moores were Dublin city planners by inheritance: the first Earl had laid out Drogheda Street (now O'Connell Street), Moore Street, Henry Street, and Mary Street. His descendants applied the same grid logic to Monasterevin, creating the parallel Main Street and Drogheda Street still visible today.

The Grand Canal arrived in 1786, connecting Monasterevin directly to Dublin and points west. The town became a transport hub overnight, and the Cassidy family built a distilling empire on the back of cheap canal freight. Cassidy whiskey and St. Patrick Cross Pale Ale travelled the world; the wealth they generated made the Cassidys the most powerful family in the town. In 1798, Cassidy was the local magistrate. That May, during the United Irishmen rising, insurgents from the surrounding countryside marched on Monasterevin in an attempt to capture it. The Battle of Monasterevin was fought in the Main Street, with local yeomanry and militiamen firing from a fortified St. John's church. The Monasterevin Yeomanry Cavalry charged and routed the insurgents. Later that year, Fr. Edward Prendergast was arrested for ministering to insurgents camped on Iron Hill near Nurney. He was hanged in the garden of Monasterevin House and buried there. A Captain Padraig O'Bierne and a group of Derryoughter boatmen later slipped into the town under cover of darkness, dug up the body, and carried it home to Harristown for proper burial.

On 2 July 1903, the Gordon Bennett Cup ran through Monasterevin. It was the first international motor race ever held on the islands of Britain or Ireland, organized by the Automobile Club of Great Britain and Ireland because racing was illegal on British public roads. Kildare was chosen as the venue partly because its roads were unusually straight, considered a safety benefit at the time. The British team faced a problem: the conventional national racing colors had already been claimed by Italy (red), Germany (white), and France (blue). As a compliment to their Irish host, the British team painted their cars Shamrock green. The color stuck. It became known as British Racing Green - the deep green still painted on Aston Martins, Lotuses, and Jaguars more than a hundred and twenty years later. The first international motor race in the British Isles, run through a Kildare town named after a sixth-century saint, gave Britain its national racing color.

Gerard Manley Hopkins - the English Jesuit poet of "The Windhover" and "Pied Beauty," the inventor of sprung rhythm, the most original lyric voice in Victorian verse - visited Monasterevin seven times in his final years. He had been posted to University College Dublin in 1884, an unhappy English Catholic in a country he never quite settled into, suffering from depression and ill health. He found respite in Monasterevin, where the Cassidys hosted him. He walked the towpaths. He looked at the aqueduct. He wrote letters home. He died in 1889, aged 44. The Monasterevin Hopkins Society holds an annual festival in his memory. The poet's quiet weekends in this canal town are part of the town's identity in a way the Cassidys would have approved of - the Venice of Ireland, hosting a Jesuit poet whose verse changed twentieth-century English. The aqueduct still carries the Grand Canal over the Barrow. The lift drawbridge on the R424 still rises and falls for canal boats. The population has more than doubled since 2002 - from 2,583 to 5,307 - as Dublin commuters discover the town. Monasterevin remains, as ever, a place defined by its waters.