Ever since the 1798 rebellion, historians, in seeking to explain its failure, have laid great emphasis on the intelligence network of agents and informers organised by Dublin Castle. Two questions have remained relatively unanswered: what did Dublin Castle know and when did it know this? This book seeks to assess the role of informers in the 1790s and the Castle's receptivity and sensitivity to material handled. Firstly, it offers a detailed assessment of the leading informers who were in constant communication with the Castle, and secondly, for the first time it provides a full, annotated text of the extant letters written during the years 1795 to 1801 by Francis Higgins, perhaps the most colourful and most secret of the Castle's correspondents. It was Higgins who was largely instrumental in the capture of Lord Edward Fitzgerald, and as the letters attest, he provided his 'handler', Under-secretary Edward Cooke, with much other vital information on the United Irish circle in Dublin during these pivotal years. The volume concludes with three appendices: Dublin Castle's own assessment of its knowledge of the United Irishmen in 1795; the letters of Higgins' chief agent, Francis Magan; and lists of United Irishmen who surrendered in Dublin in September 1798.