It was the biggest seaborne invasion in military history. Preparation for Operation Overlord, the allied invasion of Europe in June 1944, began three years beforehand. The planning was utterly meticulous. Knowledge was confined to an elite group and never seeped outside of that close circle.

Thousands of Irishmen were involved in what would become known as D-Day. Many of those were from the neutral Irish Free State. The Royal Ulster Rifles was the only regiment with two battalions in operation on the day of the invasion of Normandy itself. The 1^st battalion went in on board gliders and landed behind enemy lines, the 2^nd battalion took the more traditional route.

One Irishman centrally involved was the future Lord Killanin. He was a staff officer in an armoured division. Despite being completely ‘in the know’ about the invasion plans he was allowed back to Ireland on leave a fortnight before D-Day. A few sherries too many in the wrong place and the entire plan might have been betrayed unknowingly.

The Sweeney family, who ran the meteorological station and lighthouse at Blacksod in Mayo played their own small but significant part in proceedings. The invasion had originally been planned for 5 June but the weather was so vile it was cancelled. If it could not happen on 6th or 7th June it would have to be postponed for a number of weeks so that the moon and the tides aligned to maximum benefit. Would the weather break sufficiently to allow for the invasion to take place. It was confirmation from Edward Sweeney at Blacksod lighthouse in Mayo that there was a brief window available that prompted Eisenhower to order the operation to proceed. The temporary improvement in the weather took the Germans completely by surprise.

The commanders of the invading force expected a renewal of chemical warfare on a scale not seen since the Great War. Paddy Devlin from Galway, waiting on board a glider with the rest of the 1^st Royal Ulster Rifles, remembers being ‘issued with new suits of battle dress that were as stiff as planks and had a white residue on them.’ They were told that they had been soaked in a solution to prevent lice. In fact they were designed to counteract gas – Devlin chose to believe the story about the lice.

The greatest loss of life was suffered at one of the American beaches, codenamed Omaha – the tragedy is graphically depicted in the opening scene of Saving Private Ryan. Dubliner David McCaughey operated a landing craft at Omaha. He was hit early on in the landings and was forced to sit out most of the day on the foreshore sheltering in his stricken vehicle. He became conscious of two oddities. The first was the figure ‘one’ in red on the backs of the helmets of the dead US soldiers who had pitched forward when struck by bullets or shrapnel. This was the insignia of the 1^st Infantry Division, nicknamed, the Big Red One. The second memorable feature of the day was an odd transparent material blowing across the beach in huge quantities. McCaughey remembered seeing this wrapped around the soldiers’ guns. They had removed it when they reached the beach. He found out later that he was looking at polythene for the first time. It is a sad fact of life that war tends to spark scientific innovation.

Operation Overlord, the allied invasion of German-occupied France, took place seventy years ago, on this day.