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

Ireland is one of four members of the European Union that are not members of NATO. The others are Austria, Cyprus and Malta. The country has a longstanding policy of military neutrality: it does not join military alliances or defence pacts, or take part in international conflicts. The nature of Irish neutrality has varied over time. Ireland declared itself a neutral state during the Second World War, and during the Cold War it did not join NATO nor the Non-Aligned Movement. Since the 1970s, some have defined it more broadly to include a commitment to "United Nations peacekeeping, human rights and disarmament". Recent Irish governments have defined it narrowly as non-membership of military defensive alliances. The compatibility of neutrality with Ireland's membership of the European Union has been a point of debate in EU treaty referendum campaigns since the 1990s. The Seville Declarations on the Treaty of Nice acknowledge Ireland's "traditional policy of military neutrality". The Irish Defence Forces have been involved in many UN peacekeeping missions.