The Cathach of St. Columba is traditionally associated with St. Columba (d. AD 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 in Cairbre Drom Cliabh (now in Co. Sligo). Paleographic evidence dates the manuscript to 560-600, or a little later, but that it was written by Columba is now doubted. 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); the complete manuscript would have contained 110 folios. Rubrics written in Old Irish appear above the text of the Psalms. It may be the oldest extant Irish manuscript and may contain the earliest examples of a written Goidelic language apart from Ogham inscriptions. The maximum folio size is 270 by 190 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 guilloche 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.