Blacksod is not what most people picture when they imagine an Irish lighthouse. It is not a tall white tower on a cliff. It is a low, square, two-storey building of grey granite, with a small white conical lantern perched on top - one of only two true square lighthouses in Ireland. It was completed in 1866, built from blocks quarried at the Beaufort quarries on nearby Termon Hill, paid for substantially by a Belmullet merchant named Bryan Carey. The original proposal, in 1841, was to build it offshore on Blackrock Island. But the senior engineer of the Dublin Ballast Board, George Halpin, argued that a light at the southern tip of the Mullet Peninsula itself would do more good - guiding vessels into the shelter of Blacksod Bay rather than just warning them off a rock. He won the argument. The first lamp was lit on 30 June 1866 and has been kept burning, in one form or another, ever since.

On 1 November 1933, a man named Edward Sweeney - Ted to everyone - was appointed live-in attendant. He and his wife Maureen Flavin Sweeney would run the station, the local post office, and the daily weather observations for decades. They raised their children in the keeper's house. Maureen took the hourly readings - barometer, wind speed, wind direction, cloud cover - and Ted passed them to Dublin. During the late 1970s, the post office moved into a new bungalow up the road, but for a period it operated from one of the rooms beneath the lighthouse tower itself - the only working post office ever to share a building with a working lighthouse anywhere in Ireland. The Sweeneys were not glamorous. They were two people doing several jobs at the end of a peninsula, and the job they happened to be doing in June 1944 turned out to be one of the most consequential in twentieth-century history.

Ireland was neutral during the Second World War, but neutrality had nuances. Since independence in 1922, the country had supplied weather reports to Britain under a quiet bilateral agreement; the meteorological data was deemed too important to interrupt, and Ireland had no military secrets at stake. By June 1944, Eisenhower's planners knew that any successful Channel crossing depended on a narrow window of low winds, low seas, and broken cloud for the airborne troops. They were watching weather stations all across the northwest of Europe, but Blacksod sat farthest west - the first place that storms coming off the Atlantic could be measured. On the morning of 3 June, Maureen Sweeney's hourly readings showed a sharp drop in pressure and rising winds. Dublin queried the figures twice; she confirmed them both times. Captain Stagg looked at the data and told Eisenhower that the weather on 5 June would make a landing nearly impossible. Eisenhower asked when conditions would improve. Stagg said briefly, on the 6th. Eisenhower delayed. On 6 June 1944, 156,000 Allied troops landed at Normandy in the brief window the storm had opened. Had they landed in the storm of the 5th, history might read very differently.

For most of her life, Maureen Sweeney's role in D-Day was a footnote known only to historians and her neighbours. She kept taking weather readings. She raised her family. She lived quietly. Then, in her late 90s, recognition finally came. In 2021 the United States Congress sent her a special citation. She died on 17 December 2023 at the age of 100, some six months short of her 101st birthday; her obituary ran in The New York Times. Ted had died years before. The Blacksod Lighthouse - still operated by the Commissioners of Irish Lights, still used as a forward refuelling base for Irish Coast Guard helicopters - now opens as a visitor centre with guided tours. The keeper's house was badly damaged in a 1991 storm when waves broke through the southern sea wall, but the building was repaired, and the room where Maureen took her readings is still there. So is the barometer.

Walk to the lighthouse from the small car park above Blacksod Pier. The granite is grey under most skies and silver under cloud. The square keeper's house with its castellated tower looks more like a small fortified manor than a lighthouse. To the south, the Atlantic stretches away toward the Duvillaun Islands and beyond. To the west, Blackrock Island sits like a tooth on the horizon - the lighthouse where four Irish Coast Guard crew members died in March 2017 when their rescue helicopter crashed while trying to refuel at Blacksod. To the east, the calm waters of Blacksod Bay hold a few fishing boats at anchor. There is no monument grander than the building itself. Stand at the door where Maureen stood at 1 a.m. on 3 June 1944, look west into the wind, and try to imagine knowing that the numbers on your barometer were about to change the date of an invasion.