The work began in 1731 under Edward Lovett Pearce, the architect who was busy raising Dublin's new Parliament House and had little time for ditches. He handed the job to a young assistant named Richard Cassels, a French Huguenot who had fled religious persecution, studied navigation works across Germany and the Low Countries, and arrived in Ireland calling himself Richard Castle. When Pearce died in 1733, Castle inherited the project and the responsibility of building the first navigation lock Ireland had ever seen. He was discharged in 1736, probably because he was paying more attention to the country houses he was designing on the side. His successor, Thomas Steers, finished the job by 1741. Castle, meanwhile, went on to become one of the great architects of Georgian Ireland. The canal made him; he is barely remembered for it.

The route ran roughly twenty miles from Portadown to the Albert Basin at Newry, lifting boats over a summit 23.8 metres above the tidewater of Carlingford Lough. Fourteen locks did the work, nine of them descending to the sea. The original brick lock walls began crumbling almost immediately, and were eventually refaced with cut stone from the Benburb quarries. There were defective gates, water shortages, and a summit section judged too narrow almost from the day it opened. The manager, Acheson Johnston, had to report all of this to Parliament in 1750. None of it mattered to the merchants of Newry, who saw the canal as the lever that would lift their small river town into a major port. They were not wrong. By 1769, a separate ship canal carried sea-going vessels three and a half miles south to deep water, and Newry became the fourth largest port in Ireland.

It was built for coal, but coal almost never came. The Tyrone Navigation, the upstream link that was supposed to feed the colliery output down to Lough Neagh, dragged on through delays for half a century and never delivered the trade the engineers had imagined. What flowed instead was grain, butter for export, and bolt after bolt of Ulster linen woven in the cottages and small mills of the Bann valley. By the 1790s the canal earned around £7,000 a year in tolls. In 1813 a man named William Dawson started a passenger service from Portadown to Newry, and travellers floated where coal was supposed to. The waterway never made anyone rich. It made a great many things possible.

The railway from Belfast to Dublin opened in 1852 and ran parallel to the canal for much of its length. Traffic on the water halved within forty years. The last commercial barge tied up in 1936. On 7 May 1949 a warrant of abandonment closed most of the canal; the remaining Newry town section was abandoned in 1956. When the trust that owned it was wound up in 1974, the local councils bought the canal in chunks for two pounds each. Lock gates rotted away. The cut filled with reeds. And then something else happened. Otters moved in. The Brackagh Bog stretch became home to dragonflies, damselflies, and nineteen species of butterfly found nowhere else in Northern Ireland. Walkers came back along the towpath, then cyclists, and the route became part of the National Cycle Network's Route 9, which will eventually link Belfast to Dublin once more, the long way round.

Walk the towpath today between Portadown and Newry and the engineering is still legible in the landscape. The lock chambers, faced with Mourne granite, are mostly intact, even where the gates are gone. The summit pound holds water. The stone bridges at Scarva and Poyntzpass arch over a green linear park where coal barges once worked their way uphill. There are still proposals to reopen the navigation; none have come to anything yet. But the cut is no longer a relic in the strictest sense. It is a parkland, a wildlife corridor, and a quiet reminder that the first time someone in these islands made water flow over a hill, they did it here, in the soft country between the Bann and the lough.