Written in Latin. The Cathach is the oldest extant Irish manuscript of the Psalter and the earliest example of Irish writing. It contains a Vulgate version of Psalms XXX (10) to CV (13) with an interpretative rubric or heading before each psalm. It is traditionally ascribed to St Columba as the copy, made at night in haste by a miraculous light, of a Psalter lent to Colm Cille Columba by St Finnian. A dispute arose about the ownership of the copy and King Diarmait Mac Cerbhaill gave the judgement ‘To every cow belongs her calf, therefore to every book belongs its copy’. The arbitration failed and the Psalter of St Columba passed into the hands of the O'Donnells after the battle of Cul Dremhne in A.D. 561. St Columba went to Iona in A.D. 563. It is possible to date the manuscript to the late 6th or early 7th century from the script, but modern historical scholarship has cast doubts on St Columba's authorship.

The Psalter remained in the possession of the O’Donnells but in the custody of the Mac Robhartaigh family at Ballymagroarty, Co. Donegal. Between 1062 and 1098 a special cumdach or shrine was made for it and the manuscript was named ‘Cathach’ or ‘Battler’ from the practice of carrying it thrice right-hand-wise around the field of battle as a talisman. It was taken to France in 1691 and brought back to Sir Neal O'Donel, Newport, Co. Mayo, in 1802. The manuscript was discovered in 1813 when the cumdach was opened by Sir William Betham. It was deposited in the Academy by Sir Richard O'Donel in 1843.

The script, in the hand of a single scribe, is early majuscule with ornamental capitals, some of which are in red and, like the red in the lettering for the rubrics, the colour has faded. The framework of the capitals is often outlined by a series of scarlet dots and the decoration consists mostly of spirals and animal heads. The capitals do not stand out from the text but are drawn in by a series of letters of diminishing size.

The leaves when taken from the casket were caked together and cockled. In 1920, in the British Museum Bindery, the leaves were separated and mounted in paper frames and the butt joints were overlaid with white net. In 1980-1 further repair and rebinding work was carried out by Roger Powell and his assistant, Dorothy Cumpstey, at a cost of £6,150Stg for the repair and £250Stg for the case. The paper mounting, from which the vellum leaves had come adrift, was replaced by new vellum mounts specially stained to match the colour of the original leaves. Pieces of degreased fish skin were used for joining butted edges in the vellum mounts. The leaves, assembled in sections, were sewn within a zig-zag of hand-made paper onto cords and bound in English oak boards. The spine was covered in white alum-tawed pigskin. To keep the vellum under pressure and to prevent cockling, the rebound manuscript was put into a special box designed by David Powell and made by George Taylor in Edward Barnsley's workshop.

The Cathach was published in CD-ROM format by the Academy in 2002.

The late 11th-century shrine (cumdach) of the Cathach, made by Sitric of Kells, Co. Meath to the order of Cathbarr O’Donnell, may be seen in the National Museum of Ireland.