The dissertation argues that Ulster Catholic laity inhabited a social and cultural "frontier" through the early modern period. This mentality shaped how Ulster Catholics perceived and conceived their place and community in the rapidly changing religious, socio-economic and political situation in early modern Ulster (c.1680-1830). Though sectarian attitudes and violence are viewed as inherent in Ulster and Irish history generally, this dissertation explores the social and cultural connections between Ulster Catholics and Anglo-Scot Protestant settlers, and the social and cultural world of Ulster Catholics and Catholic converts. By examining several locales and specific Catholic families in the province, a nuanced portrait of interdenominational relationships and Catholic culture and society is forwarded. Additionally, the concerns of daily life and social connections are explored to register the amount of adaptation or resistance to the changing socio-economic and political conditions in Ulster. Moreover, the attempted Tridentine reforms of Ulster Catholic practice by the Catholic upper clergy during the seventeenth and eighteenth centuries was another "frontier" that was alternately adapted and resisted by the Ulster Catholic laity. Analysis of Catholic diocesan letters, Gaelic poetry and songs, family and estate papers, official state papers, and other contemporary works demonstrates the complexities of local interdenominational relationships and the diverse constructions of a "Catholic community" early modern Ulster.