The oldest physically surviving examples of Irish-language literacy are a few hundred inscriptions written in the Morse code–like alphabet called ogham. Almost all of these simply record names of people. They are found carved on large stones across southern Ireland (particularly Cork and Kerry) and in Irish-influenced parts of western Britain, and they date from about the fourth century C.E. to the seventh. For the invention of the ogham alphabet itself, a dating only a little earlier than that of the first extant inscriptions has been proposed, but it is certain that only a fraction of the earliest evidence has survived, so the script may have originated as soon as the influences inspiring it began to be felt in Gaeldom. The main such influence is believed to have been the Roman alphabet. This was used primarily for writing Latin (some of the stones in Britain give a Roman-letter "translation" into Latin of the Irish name that appears in ogham). By the second century C.E. at the latest, the Roman invasion of Britain had brought Latin, as a potentially culture-affecting force, to the shores of the Irish Sea (and probably across it; there is known to have been Roman trade with Ireland and arguably some transient settlement). Although Ireland was and remained outside the Roman empire, it may therefore have been as early as this that educated but hitherto illiterate Irish-speaking circles first gained the fairly minimal access to Latinity that they needed for the ogham script to be devised.
Whether or not some knowledge of Latin reached Ireland before the Christian gospel did, the language was necessarily involved in the establishment of the religion there: Irish churches could not have been part of Catholic Christendom, as they were, without using some Latin right from their foundation. Of Latin works known to have been written in early Ireland, the oldest that survive in terms of composition (not in physical terms; they are probably copies of copies) are two letters authored by the Briton Saint Patrick, probably in the fifth century. As Christianity was believed until recently to have been first introduced to Ireland by Patrick, he and his epistles have conventionally been seen as marking the necessary introduction of Latin literacy to the island as well. But not only does the ogham phenomenon precede his traditional dates, Irish Christianity is now recognized to do so too. So unless Patrick was actually active before the fifth century, Latin reached Ireland first.
Weak as Latin culture still was in Ireland in Patrick's day, his sixth-century successors established it firmly. The Latin of the writing tradition that they set up is known to have been pronounced in a markedly British fashion; thus they too were from Britain. These evangelists were doubtless inspired in many cases by the zeal that appears to have swept the larger island after the publication there of De excidio Britanniae (The ruin of Britain), a prophetic call for reformation in church and state by their compatriot Gildas. His ability to compose this erudite work a century or more after the fall of Rome shows that Latin learning was still strong in Britain at that time, and the prestige the work conferred probably played a major part in invigorating the stylish and productive British-Latin tradition that continued down to Norman times. Elements of that tradition surviving from the seventh century fall into a penitential genre, which spread to Ireland: We have Hiberno-Latin examples from the same century. By the year 700, Ireland had produced a significant body of Latin in other genres, too, that has come down to us, albeit preserved in later manuscripts.
Accomplished authors responsible include Cummian (computistics), "Augustinus" Hibernicus (theology), Virgilius Maro Grammaticus (idiosyncratic philological discourse), and Cogitosus and Muirchú (hagiography), as well as anonymous writers of poetry and legal and historical works. Indeed, the debt of Hiberno-Latin culture to the outside world was being actively repaid during that period: Seventh-century English scholars frequently traveled to Ireland for further study, while influential Irishmen such as Saint Columbanus (d. 615) had begun to spearhead a continent-wide monastic movement that did much to keep Latin learning alive on a wider stage in troubled times.
As soon as Celtic scribes began to write Latin texts on vellum, they probably included Latinized versions of Celtic names (Patrick and Gildas both did this). But the first extant manuscript material to constitute real Celtic-medium writing consists of explanatory glosses added in Irish to Latin texts penned from the early seventh century onwards. Given their ancillary nature, these physically earliest examples have been seen as reaching us from a stage not long after the actual beginning of the (manuscript) writing of Irish. However, Irish glosses in a famous Würzburg manuscript, though themselves of the eighth century, show traces of a spelling system whose invention must predate the introduction of British-Latin pronunciation by Saint Patrick's successors. Indeed, this system shows links with ogham orthography. Can it have originated in the same period? Ogham on stone was formerly felt to have been a pagan phenomenon that gave way to the Christian practice of writing manuscripts in Roman letters on vellum. But some ogham stones also display Christian crosses. Since ogham's straight strokes are ideally suited to carving, the choice of alphabet may actually have been determined more by the medium than by the culture. So the same people who carved inscriptions using ogham may also have written on vellum using the Roman alphabet. The ogham was Irish-language; any contemporary Roman-letter material will have been primarily Latin-medium, but may it have included Celtic as well? It is true that some of the stones survive while no physically contemporary manuscript texts do; but then, only ten manuscripts (in either language) went on to survive on Irish soil from even as late as 1000 C.E., and hundreds are known to have existed by then.
At all events, once Irish-medium manuscript literacy was established in a form that comes down to us, it can be seen blossoming in a variety of genres, including theological tracts, saints' lives, legal material, poetry, and ultimately the great prose tales. Early Irish literacy also displays an astonishing assurance. By the year 700 fully bilingual material was being written, showing that (uniquely for a vernacular) Irish-medium literacy was esteemed equally with Latin. It cannot be coincidence that, during the mainstream Old Irish period that followed, this literacy went on to constitute the earliest and, for its day, by far the largest body of nonclassical vernacular written material in Europe (a distinction often, but erroneously, claimed for Old English).
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Harvey, Anthony. "Problems in Dating the Origin of the Ogham Script." In Roman, Runes, and Ogham, edited by John Higgitt, Katherine Forsyth, and David N. Parsons. 2001.
Howlett, D. R. The Celtic Latin Tradition of Biblical Style. 1995.
Jackson, Kenneth. Language and History in Early Britain. 1953. Reprint, 1994.
Lapidge, Michael, and Richard Sharpe. A Bibliography of Celtic-Latin Literature, 400–1200. 1985.
McManus, Damian. A Guide to Ogam. 1991.
Stevenson, Jane. "The Beginnings of Literacy in Ireland." Proceedings of the Royal Irish Academy 89 C (1989): 127–165.