While most 21-year-olds spent the night of their birthday celebrating, Maureen Sweeney was painstakingly taking barometer and thermometer readings at the remote Blacksod weather station on Ireland’s west coast.
Her extraordinary - and unknown - role in altering the course of World War Two, saving countless lives, is detailed in a new RTE documentary.
Although Ireland may have been neutral in the Second World War this documentary sheds light on how De Valera sanctioned the passing on of weather readings to the allies which proved pivotal to the date of D-Day.
The readings from Mayo were crucial as they were the first sign of weather coming across the Atlantic two days in advance.
General Eisenhower originally planned D-day for June 5, but it was postponed after Maureen reported a dramatic drop in low pressure on June 3 pointing to an impending storm over the invasion beaches.
“She’s out on her 21st birthday at one in the morning doing a weather observation and what does she spot but the change in pressure, the pressure falling away was the red flag.”
The storm that Maureen observed coming in from the Atlantic was set to hit the Normandy beaches 500 miles away in two days on June 5, the original date for D-Day.
The documentary reveals how Eisenhower had no choice but to stall the 160,000 troops on nearly 7,000 ships, along with thousands of airmen, as stormy conditions and poor visibility would have played havoc with the landing.
But on June 4, Maureen Sweeney was back at work reading the barometer and the thermometer - crude instruments by today’s standards but critically important weather predicators.
“She 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”, said former RTE weather forecaster Ger Fleming.
Her routine was to take another dramatic turn, as on the afternoon of June 4th she began to see a rise in pressure which signalled there was going to be a clear window of weather on June 6th.
Maureen’s updated readings were passed on to General Eisenhower who gave the signal to invade on June 6.
Critically, this was also a reading which the Germans were unable to gauge as they had no weather ships in the north Atlantic so they couldn’t see the change in the pressure.
“The weather information that the Germans had in the lead up to D-Day basically suggested that a big storm was coming and it would be impossible for any allied armada to land anywhere in northern France", said 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."
The flotilla of 160,000 men by land and sea was all dependent on the meticulous readings of one west of Ireland woman. 11, 500 aircraft flew across the channel.
Susan Eisenhower, the granddaughter of the US General, said there was so much at stake.
“I think Nazi Germany could not imagine in their wildest dreams would be crazy enough to make the assault on that day.”