Since the early 1990s a significant contribution has been made to the historiography of the relationship between the Spanish monarchy and Ireland during the early modern period. In building upon the earlier pioneering studies of Brendan Jennings, John J. Silke and Micheline Kerney Walsh, recent research – such as the work of Declan M. Downey, Enrique García, Benjamin Hazard, Gráinne Henry, Patricia O’Connell, Ciaran O’Scea, Igor Pérez, Óscar Recio, Ofelia Rey and Karin Schüller – has allowed for a comprehensive understanding of this relationship and its role in Spanish and Irish history. These excellent studies tend to focus on the diplomatic, political, cultural, religious, commercial and social aspects of the relationship. However, while the military aspect of the relationship has received some scholarly attention, the features and characteristics of Irish military service to the Spanish Habsburg dynasty have yet to be subject to a detailed study.

The Spanish armies of the early modern period were formed by different ‘nations’, and it is hence surprising that historians have not yet provided a detailed analysis of any of the ‘nations’ who served the major European and indeed world power of this period. An examination of the relations between each ‘nation’ and the Crown, and how these worked in practice, can help improve our understanding of the nature of personal and informal relations, and their dynamics and significance in the evolution of the patronage systems and networks that were so important for society during the ancien régime. The study of one of these ‘nations’ in Spanish Habsburg service can help shed light on this issue, as well as creating a template for future research on all the other ‘nations’.

The first major publication regarding Irish soldiers under Spanish colours was by Brendan Jennings. His book, Wild Geese in Spanish Flanders, 1582–1700 (Dublin, 1964), is a classic piece of original research. It remains a vital reference work as it contains a vast amount of information, drawn from the Archives Générales du Royaume/Algemeen Rijksarchief in Brussels, regarding the Irish contingent in the Army of Flanders.