The Cathach of St. Columba (Dublin, Royal Irish Academy, s. n.) is an early seventh century Irish Psalter. It is traditionally associated with St. Columba (d. 597), and was identified as the copy made by him of a book loaned to him by St. Finnian, and which led to the Battle of Cúl Dreimhne in 561. Paleographic evidence, however, dates, the manuscript to the seventh century. The 58 folios in the damaged and incomplete vellum manuscript contain the text of Psalms 30:10 to 105:13 in Latin (the Vulgate version). Rubrics written in Old Irish appear above the text of the Psalms. It may be the oldest known Irish manuscript and may contain the earliest examples of a written Goidelic language apart from Ogham inscriptions. The maximum folio size is 200 by 130 mm.
The decoration of the Cathach is limited to the initial letter of each Psalm. Each initial is in black ink and is larger than the main text. They are decorated with trumpet, spiral and guilloch patterns and are often outlined with orange dots. These patterns are not merely appended to the letters or used to fill spaces. They instead distort the shape of the letters themselves. The letters following the enlarged initials gradually reduce in size until they reach the same size as the main text. Although the motifs of the Cathach decoration are not similar to decorations in later manuscripts, such as the Book of Durrow (which followed the Cathach by as many as seventy years), the ideas of decoration which distorts the shape of the letters and the diminution of initial letters are ideas which are worked out in great detail in later Insular art.
The Cathach was enclosed in a shrine in the eleventh century by Cathbar O'Donnell head of the O'Donnell Clan and Domnall McGroarty Abbot of Kells. The shrine was carried into battle by the McGroartys as a talisman, consistent with its psalter's origins starting the Battle of Cúl Dreimhne between saints Finnian of Moville and Columba (hence the name: Cathach = "Battler"). The manuscript was rediscovered in 1813, when the shrine was opened. The Cathach was entrusted to the Royal Irish Academy in 1842 by Sir Richard O'Donnell. The O'Donnell family has since claimed ownership of the Cathach but the manuscript remains in the custody of the McGroartys, its official "Keepers". The Cathach's shrine is now in the National Museum of Ireland, Dublin.