It is hard to imagine that a remote, windswept part of Co Mayo was the setting for an event 75 years ago that changed the course of history.

Yet, for a few tense days in June 1944, the success of the greatest military invasion the world had ever seen depended on weather readings taken by a 21-year-old Irishwoman at the remote Blacksod weather station on the Mullet peninsula in Co Mayo.

The resulting Operation Overlord — D-Day — was a major turning point in the Second World War.

In the largest amphibious military landing ever, some 160,000 Allied troops landed on multiple beachheads in France to fight the Nazis.

June 3, 1944, was a day of personal significance for Maureen Flavin, as she then was.

It was her 21st birthday but, instead of celebrating with friends, she was busy taking barometer readings and collating weather reports by Ted Sweeney, the lighthouse keeper at Blacksod and the man she would later marry.

Maureen sent her readings to Ballina, Co Mayo, where they were sent to Dublin, then to the UK meteorological office in Dunstable, 50km north of London.

The commander of Allied forces, General Dwight D Eisenhower, had originally planned D-Day for June 5 but Maureen’s data threw his planned invasion strategy into chaos.

According to her readings, a storm coming in from the Atlantic was set to hit the Normandy beaches 800km away in two days on June 5.

Eisenhower was forced to mediate between opposing US and British weather advisers and generals.

Nearly 5,000 ships and more than 11,000 aircraft would carry the troops into battle on the day across a 90km beachfront and into the interior of the Cotentin peninsula in Normandy.

Because of the importance of the landings by sea and by air, June 6 and 7 were also pinpointed as possible dates because moon and tide conditions were suitable.

Calm weather was needed for the landings and for air support.

The troops would be carrying heavy backpacks, so if they jumped out of the boats in bad weather they would be turned over very quickly by strong waves.

They needed clear skies for the bombers so they could see their targets.

Without calm seas and clear skies, the invasion would have had to be delayed for weeks, giving Hitler’s troops time to prepare.

The weather readings she took at Blacksod, Co Mayo.

Eisenhower decided to postpone the invasion and prayed for better weather.

Less than 24 hours later, his prayers were answered by Maureen, who reported a rise in pressure which signalled there was going to be a clear window of weather on June 6.

“I think Nazi Germany could not imagine in their wildest dreams would be crazy enough to make the assault on that day,” says the US general’s granddaughter, Susan Eisenhower.

Maureen’s updated readings were passed on to General Eisenhower, who gave the signal to invade on that day.

The Nazis were taken by surprise, as they had no weather ships in the north Atlantic and couldn’t see the pending improvement in the weather.

“The weather information that the German had in the lead up to the D-Day basically suggest that a big storm was coming and it would be impossible for any Allied armada to land anywhere in northern France,” says Robert Gerwart, professor of Modern History at UCD.

“Lots of senior officers, including Rommel himself, took a few days off. Rommel went to Germany for his wife’s birthday so, in this context, the much more accurate weather reading the Allies had proved to be a huge advantage.”

Maureen’s role in altering the course of the war is detailed in a RTÉ documentary to be screened on Thursday, June 6 — the 75th anniversary of D-Day.

D-Day veteran Joe Cattini

In it, 96-year-old D-Day veteran Joe Cattini says the 160,000 men who landed in Normandy on D-Day owed a debt of gratitude to her.

“We owe a lot to Maureen of the west of Ireland, us who invaded France on D-Day, because if it hadn’t been for her reading of the weather we would have perished in the storms,” says Joe.

It was not until the invasion was well underway that Maureen understood the implications of her work.

“She was completely oblivious to the decisions that were going to be taken in the next while that were at least partly dependent on what she was doing”, says Ger Fleming, former head of forecasting at Met Éireann.

Although Ireland remained neutral in the war — or ‘The Emergency’ as it was known here — it was a form of benevolent neutrality as Éamon de Valera authorised the passing on of weather reports to the Allies, while refusing them to the Nazis.

Maureen had never planned to be in the weather forecasting business.

“I meant to go to America but I didn’t get that far so I went for a job in the post office instead,” she said.

Speaking to the Irish Examiner, her son, Vincent, describes the journey she took at the age of 20 from her home in Knockanure, north Kerry, to Blacksod in Mayo.

“My grandmother put an ad in the paper about a vacancy for a post office clerk and my mother answered it and came up,” he says.

It took her two and a half days to get there, including one overnight in Limerick, another in Ballina in Mayo, and then the mail car all the way out here. That was how she met my dad.

Vincent joined the lighthouse service in 1974, working offshore and taking over from his father as lighthouse keeper at Blacksod in 1981.

“My dad’s mother, Margaret Sweeney, had the post office. Another aunt of mine, dad’s sister, Francis, also helped out with the weather reports that my father collated.”

It was only when Maureen got to Blacksod that she realised part of her duties was to carry out weather observations for the Irish Met service, but she took it all in her stride.

“There was no learning in that,” Maureen recalls. “You fell into it automatically. I didn’t like the night duty, though. I used to be afraid that the Germans would come in at night and come ashore.”

Thanks in part to her efforts, that never happened.

Storm Front in Mayo — The Story of the D-Day Forecast will be shown on RTÉ One on Thursday June 6, at 10.15pm.