The Book of Fenagh (Irish: Leabar Fidhnacha) is a manuscript of prose and poetry written in Classical Irish by Muirgheas mac Pháidín Ó Maolconaire in the monastery at Fenagh, West Breifne (modern-day County Leitrim).^[1] It was commissioned by Tadhg Ó Rodaighe, the coarb of the monastery, and is believed to derive from the "old Book of Caillín" (Irish: Leabar Chaillín), a lost work about Caillín, founder of the monastery.^[1] Ó Maolconaire began work about 1516.^[1]

The O'Roddy coarbs and descendants retained the book down to Brian O'Roddy, parish priest of Kilronan (Ballyfarnon) in the mid-19th century, upon whose death it was retained by his successors as parish priest.^[2] It later passed to George Michael Conroy, Bishop of Ardagh and Clonmacnoise, for safe-keeping, before his successor Bartholomew Woodlock sold it in 1888 to the Royal Irish Academy (RIA) for £10.^[3] This was on the advice of Denis Murphy, a Jesuit, that the RIA were best able to preserve it.^[3][4] Its catalogue number is RIA MS 23 P 26: Cat. No. 479.^[1]

John O'Donovan made a facsimile transcript in 1828, and a manuscript English translation in 1830.^[5] The first published edition was in 1875, edited by William Maunsell Hennessy and translated by Denis H. Kelly from O'Donovan's facsimile.^[5] The Irish Manuscripts Commission published a supplementary volume in 1939 with material missing from previous versions.

Marginal notes in Irish adorning the book are commentaries by the noted Irish antiquarian Tadhg O'Rodaighe (floruit 1700) from Crossfield in Fenagh. The book was used as a source for the Annals of Connacht and the Annals of the Four Masters.^[5]

A metal cumdach (a book-shaped shrine) known as the "Shrine of Caillín" was built before 1536 to hold and protect the manuscript.^[8][9] Caillín is described by Lucas as "something of a specialist in the production of battle talismans"^[10] and according to legend, in his lifetime commissioned a number of battle standards, including this shrine.^[10] The shrine was badly damaged in a 2009 fire at St Mel's Cathedral, Longford, where it had been kept since 1980.^[8] It was acquired by the National Museum of Ireland the following year, while the manuscript is in the collection of the Royal Irish Academy.^[11] There is some doubt as to if the shrine was actually intended as a cumdach, given that it is smaller than the manuscript.^[8]