Since the proposition of the social integration theory by émile Durkheim, macro-sociological changes have been speculated to affect suicide rates. This study investigates the effect of the First World War on Irish suicide rates. We applied an interrupted time series design of 1864–1921 annual Irish suicide rates. The 1864–1913 suicide rates exhibited a slow-rising trend with a sharp decline from the year 1914 onwards. The odds for death by suicide for males during the 1914–1918 period was 0.811 (95% CI 0.768–0.963). Irish rates of suicide were significantly reduced during the First World War, most notably for males.