This thesis examines how Irish Catholic and nationalist military participation in the Great War reflected colonial tensions between Ireland and Great Britain in the early twentieth century. Whereas prior scholars have tended to examine Irish Catholic support for the war by studying soldiers’ combat experience or civilian attitudes on the home front, my thesis instead takes a social and cultural approach by studying the attitudes of the soldiers themselves. In chapter one, I examine how Irish Catholic soldiers of the First World War justified their service in the British forces by drawing on nationalist myth which had developed over the preceding centuries. This ‘myth of the Irish soldier’ claimed that Irishmen possessed the natural attributes of the ideal soldier. By the outbreak of World War I, this myth had been repeatedly repurposed in moderate nationalist rhetoric to suit changing circumstances, and it could thus be drawn upon to encourage Irish Catholics to fight on the side of Britain. In chapter two, I argue that the combination of this myth with rhetoric emphasizing Irish unity and sacrifice, regardless of the soldiers’ religious or political affiliation, increased solidarity among Irish soldiers within the ranks. As a result, national distinctions and tensions between Irish and English soldiers were more prevalent than inter-Irish divisions within the army, although this camaraderie among Irish soldiers was challenged by the Easter Rising of 1916 and its aftermath. This thesis indicates the complexity and malleability of religious, political, and national identity in Ireland in the early twentieth century, and it suggests that the Great War triggered a shift in identity and communal loyalty among Irish soldiers.