The subject of St. Patrick's Purgatory, the visits paid to it by pilgrims from distant lands, the visions seen therein by them, and the literature that subsequently and consequently arose, is of extreme interest. When I commenced to study the subject for my own edification and entertainment - the idea of writing a book had not then occurred to me - I was astonished to discover how much strange and out-of-the-way information might be gathered together, and yet how little knowledge my fellow-countrymen possessed of the past history of a place that was once of European fame. The only book that dealt with it in anything approaching a satisfactory manner was Canon O'Connor's St. Patrick's Purgatory, Lough Derg; of this I have used the revised edition published in Dublin in 1910. I differ from him on one or two points; but I must admit my indebtedness to him for many allusions and suggestions which otherwise I might easily have overlooked. He has written a most useful popular history of Lough Derg and the surrounding district, and has given a very good account of the later period down to the present day, which I have consequently passed over very rapidly. I have dealt with the subject on an entirely different plan, as the most superficial comparison will show, and have endeavoured to present the reader with a fairly exhaustive account of an almost forgotten episode in the ecclesiastical and social history of Ireland. For the profound scholar I have not attempted to write, but as there are many bypaths in the history of Ireland as yet almost untrodden, perhaps I may be pardoned for venturing down one of them with somewhat halting steps.<br><br>The authorities of which I have made use are sufficiently referred to in the notes. I should state, however, that Father Delchaye's most valuable article in Analecta Bollandiana, tom. XXVII, has been for me, as it will be for all other writers on the subject, a basis and a starting-point.