Maureen Sweeney delivered the weather forecast that convinced the Allied Forces to wait 24 hours before launching D-Day. 

There was a myriad of elements at play in making the Allied D-Day attack the turning point of WWII: over 160,000 Allied troops, 5,000 ships, 13,000 aircraft. 

And one 21-year-old Irishwoman named Maureen Sweeney. She is the subject of the forthcoming documentary "Storm Front in Mayo," which will air in Ireland on June 6, the D-Day anniversary, on RTE1. 

Maureen and her husband, Ted, were in charge of Blacksod Lighthouse, which presides over the Atlantic near Mayo's most westerly point. One of the roles of lighthouse keeper was to take weather reports. 

On the night of June 3, 1944 - also Maureen's birthday - they took a report indicating bad weather ahead. The winds were at force six and the barometer was rapidly dropping, indicating a storm approaching. They sent it in and, Maureen recalls, quickly received a call from a British woman asking them to check and confirm the report. 

“We took the weather in the ordinary way and passed it on. We got a message back again saying, ‘Please check and repeat’. So I went back but I was right, the barometer had dropped," she told the Irish Sun in a 2016 interview. 

An hour later, she received a call from the same British woman, asking her to check and confirm again, which she did. 

Maureen didn't know this at the time, but the Allied leaders gathered in London were relying on her weather reports to judge whether they should proceed with the D-Day launch as planned. The chief meteorologist, a Scottish man named James Scagg, was giving General Eisenhower regular weather updates. 

While Ireland was technically neutral during WWII, these weather reports were one of the covert ways in which they supported the Allies. Given the Nazi plans to invade Ireland, which never came to fruition, this was in Ireland's interest. 

Eisenhower had wanted to commence the Normandy and Omaha landings on June 5, but based on the impending storm report decided to hold back - the storm passing over Blacksod would reach the shores of Normandy on June 5, and would have created treacherous conditions at sea and poor visibility from the skies. 

On the morning of June 4, Maureen reported good visibility and an interlude in the storm, giving the Allied forces the window they needed to launch D-Day on June 6. 

Sweeney, now 96, still lives in Mayo, in a retirement home. She eventually became aware of just how significant her weather reports were:

"I was reading a piece in an English paper recently and it said that if Eisenhower had gone to war that night (June 3, 1944) it would have smashed America," she said in a 2016 interview with the Irish Independent

"That weather report from Blacksod - of all places - changed the war. Who would have thought it?"