This impressively researched and compellingly argued book successfully demonstrates the vitality and complexity of early modern Irish Catholicism through a study of Jansenism. Thomas O'Connor argues that the particular political and religious circumstances of post-Reformation Ireland facilitated the emergence of ‘pre-Jansenist’ and Jansenist varieties of Catholic reform in tandem with similar developments elsewhere in western Europe, notably France and Flanders. Jansenism, therefore, was not simply a French or Flemish import into Ireland. In fact, Irish theologians in Leuven and elsewhere contributed to the emergence of what would become Jansenism, which facilitated its spread among the growing number of Irish clergy and clerical students on the continent, as well its influence on Irish Catholic ecclesiastical structures and pastoral practices as they were remodelled in the seventeenth century. O'Connor skillfully highlights the role of several leading Irish clerics, especially Peter Lombard, but also Florence Conry and Luke Wadding. He also provides detailed studies of two leading Irish Jansenists, John Sinnich, a Leuven-based theologian, and John Callaghan, who was active in France, as well as devoting attention to the Jansenist and anti-Jansenist factions among the Irish in Paris and Rome in the mid-seventeenth century. The final section of the book argues that Irish Jansenism was particularly influential in the 1640s, when the establishment of a de facto government by the Confederate Catholics meant that the Gallican-Jansenist approach adopted by John Callaghan and like-minded clergy was uniquely suited to the political strategies of those confederates, like Donough MacCarthy, Viscount Muskerry, who would seek accommodation with Protestant royalists in Ireland. O'Connor skillfully illustrates how Jansenism impacted on and was adapted to the complexities of Irish politics in the 1640s, especially after the arrival of the papal nuncio Rinuccini in 1645. However, the high point of Gallican-Jansenist influence also occurred as Confederate Catholic unity disintegrated in the later 1640s and O'Connor traces the consequences on to the continent in the 1650s. The Restoration, O'Connor argues, witnessed a brief renewal of Irish Jansenism in the form of Peter Walsh's Gallican Remonstrance, which ultimately failed to garner sufficient clerical support in Ireland or the continent. While O'Connor is surely correct to point to the increasing Roman orientation of Irish Catholic ecclesiastical structures from the 1660s, there is still more to be said about the individual Irish men and women drawn to Jansenism until well into the eighteenth century. Thomas O'Connor has been a leading figure in the recent renewal of scholarly interest in Irish migration to early modern Europe. He has already produced an important body of work on the subject, notably his book on the eighteenth-century Irish theologian Luke Joseph Hooke. Irish Jansenists is a more ambitious study. It reveals, in an innovative way, the European dimensions of Irish Catholicism through a series of overlapping political, social, ecclesiastical and intellectual networks. It is informed by an extremely rich, multilingual primary source base, too often missing from Irish history, which draws on material in Irish, English and continental archives and libraries. The end result is a sophisticated and intricate work which constitutes a major contribution to early modern Irish, and indeed European, history.