Plan W

Plan W, during World War II, was a plan of joint military operations between Ireland and the United Kingdom devised between 1940 and 1942, to be executed in the event of an invasion of Ireland by Nazi Germany.

Although Ireland was officially neutral, after the German Blitzkriegs of 1939-40 that resulted in the defeat of Poland, the Low Countries, and France, the British suspected that Germany planned an invasion of Britain (Operation Sea Lion), and were also concerned about the possibility of German plans to invade Ireland. German planning for Operation Green began in May 1940, and the British had intercepted communications about it starting in June 1940. The British were interested in securing Ireland as its capture by German forces would expose their western flank, and provide a base of operations for the Luftwaffe in the Battle of the Atlantic and in any operations launched to conquer Great Britain as part of Operation Sea Lion.

British-Irish cooperation was a controversial proposal for both sides as most of the Irish political establishment had been combatants in the Irish War of Independence between 1919 and 1921. However, due to the threat of German occupation and seizure of Ireland and especially valuable Irish ports, Plan W was developed. Northern Ireland was to serve as the base of a new British Expeditionary Force that would move across the border to repel the invaders from any beach-head established by German paratroopers. In addition, coordinated actions of the Royal Air Force and Royal Navy were planned to repel German air and sea invasion. According to a restricted file prepared by the British Army's "Q" Movements Transport Control in Belfast, the British would not have crossed the border "until invited to do so by the Irish Government," and it is not clear who would have had the operational authority over the British troops invited into the State by Éamon de Valera, but it is assumed the British would have retained command. The document added that most people in Ireland probably would have helped the British Army, but "there would have been a small disaffected element capable of considerable guerrilla activities against the British."

By April 1941, the new BTNI commander, General Sir Henry Pownall extended his planning for a German invasion to cover fifty percent of the entire Irish coastline. He believed that German troops were likely to land in Cork, Limerick, Waterford, Westport, Galway, Sligo, and Donegal, i.e. on the southern or western coasts. British Army personnel also carried out secret intelligence gathering trips to glean information on the rail system south of the border.