During the First World War approximately 200,000 Irish men and 5,000 Irish women served in the British armed forces. All were volunteers and a very high proportion were from Catholic and Nationalist communities. This book is the first comprehensive analysis of Irish recruitment between 1914 and 1918 for the island of Ireland as a whole. It makes extensive use of previously neglected internal British army recruiting returns held at The National Archives, Kew, along with other valuable archival and newspaper sources. There has been a tendency to discount the importance of political factors in Irish recruitment, but this book demonstrates that recruitment campaigns organised under the auspices of the Irish National Volunteers and Ulster Volunteer Force were the earliest and some of the most effective campaigns run throughout the war. The British government conspicuously failed to create an effective recruiting organisation or to mobilise civic society in Ireland. While the military mobilisation which occurred between 1914 and 1918 was the largest in Irish history, British officials persistently characterised it as inadequate, threatening to introduce conscription in 1918. This book also reflects on the disparity of sacrifice between North-East Ulster and the rest of Ireland, urban and rural Ireland, and Ireland and Great Britain.