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.

The policy of neutrality was adopted by Ireland's Oireachtas at the instigation of the Taoiseach Éamon de Valera upon the outbreak of World War II in Europe. It was maintained throughout the conflict, in spite of several German air raids by aircraft that missed their intended British targets, and attacks on Ireland's shipping fleet by Allies and Axis alike. Possibilities of both German and British invasions were discussed in the Dáil. Both eventualities were prepared for, although most detailed preparations were done with the Allies under Plan W. De Valera's ruling party, Fianna Fáil, supported his neutral policy for the duration of the war.