St. Patrick’ s Purgatory, Lough Derg has long been an important site of pilgrimage in Ireland having caught the attention of the literary world in the twelfth century. This pilgrimage has continued to exist and develop throughout some of the most turbulent times cf Irish history and stands as a monument to Irish Catholicism. In this thesis I have looked at how this pilgrimage survived and developed in the eighteenth and nineteenth centuries when faced with much difficulty and opposition from many ranks of society. In order to deal with these issues effectively, 1 have divided this thesis into three chapters. The first chapter deals with the period o f 1700-1780. During this period. Ireland was subject to the Penal legislation, which placed serious limits on the practice of Catholicism in Ireland. In this chapter I have looked at the effects of the Penal legislation on Lough Derg and its pilgrimage and how in spite of these laws the pilgrimage continued often with the support of the local Protestant community of the Parish of Templecarne. I have also looked at the topography of the island in the eighteenth century and at the exercises performed by the pilgrims at this time. The second chapter deals with the period of I 780- 1850. Over this period of time, St Patrick’ s Purgatory was to witness many changes. The Clogher Diocesan clergy took over the running of the pilgrimage and much development of its facilities took place. New Churches and accommodation for the pilgrims were built during this time. This chapter also deals with the impact of the Great Famine on this pilgrimage. Chapter three deals with this pilgrimage tradition in the post famine period. It looks at the ‘devotional revolution", which swept Ireland at this time and its impact on this historical pilgrimage. This chapter also looks at the further development of the island and the pilgrimage exercises. In spite of the many hardships it faced in the eighteenth and nineteenth centuries the pilgrimage to Lough Derg continued and it maintained its esteemed position in society. I believe this sacred site and its pilgrimage tradition a worthy topic of study.