John D'Arcy inherited the Clifden estate in 1804 - mostly fishermen and farmers scattered across rough land. By 1812 he had a plan: build a proper coastal town from nothing. Bad roads and lack of capital held the project back until the potato crop failed in 1821-22 and D'Arcy petitioned Dublin for help. The engineer Alexander Nimmo arrived in 1822, built the quay (finished 1831), and started the road to Galway. The town grew explosively. By the time D'Arcy died in 1839, Clifden had 185 dwellings, two churches, two hotels, three schools, a courthouse, a jail, a distillery, and 23 pubs. The population had reached 1,100. The triangle of streets visible today was already laid out. In 1843 Daniel O'Connell held one of his Monster Meetings in Clifden, with reportedly 100,000 people gathered to hear him speak on repeal of the Act of Union. Two years later the Great Famine arrived, and by 1848 ninety percent of the population was on relief. The town survived. Many of its people did not.

In 1905 Guglielmo Marconi chose a site at Derrygimlagh Bog, four miles south of Clifden, for his first high-power transatlantic wireless telegraphy station. He needed the shortest possible distance to its sister station at Glace Bay, Nova Scotia. The Clifden station opened for public service on 17 October 1907 with the transmission of 10,000 words across the Atlantic. The technology was new enough that the first regular commercial messages between Europe and North America passed through a peat bog in Connemara. Up to 200 people worked at the station at peak. Among them was a young telegrapher named Jack Phillips, who later took a job at sea and died as chief radio operator on the Titanic, still transmitting distress signals as the ship went down. The wireless station also drew an Italian-American visitor: Marconi himself, who maintained a personal interest in the operation.

On 15 June 1919, the first non-stop transatlantic flight in history ended in the Derrygimlagh bog right next to Marconi's station. Captain John Alcock and Lieutenant Arthur Whitten Brown had taken off from St John's, Newfoundland the previous afternoon in a modified Vickers Vimy bomber. Sixteen hours later, fuel almost gone, they spotted the green of the bog through breaking cloud. Alcock thought he was looking at a meadow. The Vimy's landing gear sank into the soft peat and the nose pitched forward. Both men climbed out with only minor injuries. They were taken to Clifden by stagecoach. When they returned later to recover what they could of the airplane, locals had already helped themselves to souvenirs. Alcock and Brown collected the Daily Mail's 10,000-pound prize. The first true transatlantic crossing by air had ended in a Connemara bog within sight of the antenna that had been throwing words across the same ocean for twelve years.

On Bloody Sunday, 21 November 1920, IRA gunmen shot eleven British officers and civilians in Dublin in a coordinated morning attack. That afternoon, British Auxiliaries opened fire on a Gaelic football crowd at Croke Park, killing twelve. Thomas Whelan, born in Clifden in 1899, was arrested and charged with one of the assassinations despite witnesses placing him elsewhere. He was hanged on 14 March 1921. Two days later the West Connemara flying column of the IRA, following its 'two-for-one' policy of killing two RIC constables for every Republican executed, shot Constables Reynolds and Sweeney at Eddie King's Corner. The RIC used the Marconi station to call for help. In the early hours of St Patrick's Day, 17 March 1921, a trainload of Black and Tans arrived on the Galway-to-Clifden railway and proceeded - in the contemporary phrase - to 'burn, plunder and murder.' They killed one civilian, seriously injured another, and burned fourteen houses. Clifden remembered. A ceremony commemorates Thomas Whelan in his native town every year.

On 12 September each year, the Mexican flag flies in Clifden. The man it honors is John Patrick Riley, born here around 1817. He emigrated to the United States, enlisted in the U.S. Army, and during the Mexican-American War deserted - he said because of anti-Catholic persecution by U.S. officers. He joined the Mexican Army and rose to major in the Saint Patrick's Battalion, a unit of about 200 European immigrants, mostly Irish, fighting on the Mexican side. The battalion's flag bore the harp and shamrock. Riley survived the war. Mexico considers the San Patricios heroes. The United States considers them deserters. Clifden, which gave Riley a bronze street sculpture and an annual flag-raising, makes its own decision. The railway closed in 1935. The Marconi station was burned in 1922 by Anti-Treaty forces during the Civil War. The airport built at Cloon in 2008 has never opened. But on a clear evening on the Sky Road, with the Twelve Bens behind you and the Atlantic running west to Newfoundland, Clifden is exactly the town John D'Arcy hoped to build.