The Codex Usserianus Primus, an Old Latin Gospel Book preserved in Dublin at Trinity College Library, also known as the Ussher Gospels, is thought to have been produced in Ireland about 625, and may be the earliest surviving Irish codex. Even though it has only one decorated leaf it is also the earliest surviving example of an Insular artist copying a Mediterranean form of decoration, and it represents the beginning of the Insular illuminated manuscript tradition. The manuscript is damaged, with the vellum leaves fragmentary and discolored. The remains of the approximately 180 vellum folios have been remounted on paper.
"The manuscript has a single remaining decoration, a cross outlined in black dots at the end of the Luke (fol. 149v). The cross is between the Greek letters alpha and omega. It is also flanked by the explicit (an ending phrase) for Luke and the incipit (first few words) for Mark. The entire assemblage is contained within a triple square frame of dots and small "s" marks with crescent shaped corner motifs. The cross has been compared to similar crosses found in the Bologna Lactantius, the Paris St. John, and the Valerianus Gospels. Initials on folios 94, 101 and 107 have been set off by small red dots. This represents the first appearance of decoration by "dotting" around text, a motif which would be important in later Insular manuscripts" (Wikipedia article on the Codex Usserianus Primus).
A digital facsimile of the Codex Usserianus Primus is available from Trinity College Dublin at this link.
Alexander, Insular Manuscripts 6th to the 9th Century (1978) No. 1.