Over the course of the 20th century Ireland moved from being a largely young population with a high death rate from infectious diseases to an increasingly older population with a consequent rise in chronic diseases. Understanding the changes that occurred in Irish mortality over the 20th century and how these changes compare with those experienced by similar countries can help us plan for the challenges of our aging population. This paper analyses trends in mortality in Ireland over the period 1901 to 2006 by age group, gender and five broad categories of cause of death – infectious diseases, circulatory diseases, respiratory diseases, cancer and external causes. To place the changes in an international context the trends are compared with those experienced by Northern Ireland and England and Wales. Ireland experienced the fastest improvements in mortality of the three regions in the early years of the 21st century. By 2006 the mortality of Irish males ranked between that of Northern Ireland and England and Wales while Irish females experienced the lowest mortality of the three regions. The improvement in Irish mortality in the 21st century can be attributed mainly to the drop in deaths from circulatory diseases for both males and females.