The manuscript was thought to have belonged to St. Patrick and, at least in part, to be a product of his hand. Research has determined that at least part, if not all, of the manuscript was the work of scribe named Ferdomnach of Armagh (died 845 or 846). Ferdomnach wrote the first part of the book in 807 or 808.
There are 221 folios of vellum. It measures 7.75 inches by 5.75 inches. The text is written in two columns in a fine pointed insular minuscule. The manuscript contains four miniatures, one each of the four Evangelist's symbols. Some of the letters have been colored red, yellow, green, or black. The manuscript is associated with a tooled-leather satchel, believed to be of great antiquity.
The manuscript contains important early texts relating to St. Patrick. These include two Lives of St. Patrick, one by Muirchu Maccu Machteni and one by Tirechan. Both texts were originally written in the seventh century. The manuscript also includes other miscellaneous works about St. Patrick including the "Liber Angueli" (or the Book of the Angel)., in which St. Patrick is given the primatial rights of Armagh by an angel. Some of these texts are in Gaelic and are the earliest surviving continuous prose narratives in that language. The only Gaelic language texts of greater age are some fragmentary glosses found in manuscripts on the continent.
The manuscript also includes significant portions of the New Testament including the Epistles of St. Paul, the Epistles of James, Peter, John, and Jude, the Book of Revelations, and the Gospels of Matthew, Mark, John, and Luke. There is also prefatory matter including prefaces to Paul's Epistles (most of which are by Pelagius), the Canon Tables of Eusebius, and St. Jerome's letter to Damasus. The manuscript closes with the "Life of St. Martin of Tours", by Sulpicius Severus. The New Testament texts are based on the Vulgate, but with variations characteristic of insular texts.
The people of medieval Ireland placed a great value on this manuscript. It was one of the symbols of the office for the Archbishop of Armagh. The custodianship of the book was an important office that eventually became hereditary in the MacMoyre family. It remained in the hand of the MacMoyre family until the late seventeenth century. By 1707 it was in the possession of the Brownlow family of Lurgan. It remained in the Brownlow family until 1853 when it was sold to the an Irish antiquary, Dr. Reeves. In 1853, Reeves sold it to the Anglican Primate of Ireland, who presented it to Trinity College.