Allowing that he held the offices of fer leigind ("man of learning"; lector) and airchinnech (superior) of Ros Ailithir (Roscarberry, County Cork), Airbertach mac Cosse’s reputation as a scholar, among present-day historians, rests mainly on four surviving works on the basis of which he has been viewed as a Latinist, a commentator on the Psalms, and a poet who utilized geographical and biblical themes. Little is known of his background. His genealogy is unknown, although it seems reasonable to conclude that his origins lay in Munster. He may have belonged to the minor population group of Ui Dobrain, which features among the subject peoples of Dal Messin Corb—a Laigin dynasty—and is associated with other lineages which had mid-Munster connections. He joined the community of Ros Ailithir, which included among its founding-fathers Fachna of Corco Lofgde and Colman Ailithir (the pilgrim). The reputation for Latin learning which the foundation enjoyed is reflected in the Old Irish Triads. During his time as fer leigind, in 990, the site was attacked by a Hiberno-Scandinavian force (probably from Waterford), which carried him off as hostage. He was ransomed on Inis Cathaig (Scattery Island, County Clare) by the powerful Munster high king, Brian Boruma (Boru). Subsequently, he became superior of his community. He died in 1016.
The four surviving works most widely associated with Airbertach are found in the manuscript compilation Rawlinson B 502, at the Bodleian Library, Oxford, which, O Riain argues, is to be identified with the Book of Glendalough. These include a compound tract, written in 982, the principal concern of which is a study of the Psalms. One verse, seemingly an interpolation, refers to Airbertach as having translated some of the subject matter from Latin to Irish. There is also a lengthy poem, with a geographical theme, based on the writings of Orosius and Isodore of Seville, which is expressly ascribed to "in fer leigind Mac Coise." The two remaining poems, one dealing with the kings of Judah and the other with a battle in which the Israelites defeated the Midianites, are assigned to Airbertach because they are found in conjunction with the above-discussed compositions in Rawlinson B 502. Although it is not unreasonable to attribute these biblical poems to Airbertach, the possibility remains that they were produced by one of his students—or at least by another Ros Ailithir-based scholar.
Another work which features in the same manuscript compilation, the biblical opus known as "Saltair na Rann," may also, in the view of Gearoid MacEoin, have been composed by Airbertach. Widely regarded as one of the finest examples of Middle Irish verse, this apparently unfinished epic has been dated to 988 on the basis of a chronological passage which, it seems, formed part of the original poem. Various arguments advanced by MacEoin on this matter, including the suggestion that the poem’s incomplete state reflects a suspension of Airbertach’s work following his capture by the Norsemen in 990, have drawn opposition from Carney and others.
