Cathach of St. Columba

The Cathach of St. Columba (Dublin, Royal Irish Academy, RIA MS 12 R) is a late 6th century Insular psalter.
"An Cathach" (meaning "the Battler") was a relic used by the Clan Ó Domhnaill (O’Donnell Clan), the old Gaelic royal family in Tír Chonaill, as a rallying cry and protector in battle. It is the oldest surviving manuscript in Ireland, and the second oldest Latin psalter in the world.
The manuscript.
The manuscript is 27 cm x 19 cm and contains the Vulgate version of Psalm 30:13 to 105:13. The manuscript is dated to 560-600. The current manuscript is 58 folios; the complete manuscript would have contained around 110 folios.
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. Irish manuscripts like this one, produced in a culture isolated from Rome, were written in localized scripts. 
History.
 was used as a rallying cry and protector in battle. It was said to protect and guarantee victory in war to the Donegal leaders. Before a battle it was customary for a chosen monk/holy man (usually attached to the McGroarty clan, and someone who was sinless) to wear the Cathach in its cumdach, or book shrine, around his neck and then walk three times around the troops of O'Donnell.
As a chief of the O'Donnells, the manuscript was inherited by Brigadier-General Daniel O'Donnell (1666–1735), and was regarded by him, in accordance with its traditional history, as a talisman of victory if carried into battle by any of the Cinel Conaill. He served James II of Ireland and then in the French Irish Brigade. He placed it in a silver case and deposited it for safety in a Belgian monastery, leaving instructions in his will that it was to be given up to whoever could prove himself chief of the O'Donnells. Through an Irish abbot it was restored to Sir Neale O'Donnell, 2nd Baronet, of Newport House, County Mayo, in 1802. His son, Sir Richard Annesley, entrusted the relic to the Royal Irish Academy in 1842. The leaves were stuck together until separated at the British Museum in 1920; the manuscript was further restored in 1980–81.
The specially made cumdach is in the National Museum of Ireland – Archaeology. The initial work on the case was done between 1072 and 1098 at Kells, but a new main face was added in the 14th century with a large seated Christ in Majesty flanked by scenes of the Crucifixion and saints in gilt repoussé (NMI R2835, wide). The cumdach (book shrine) cover consists of a wooden box measuring long, wide and thick.