Early Irish literature, is commonly dated from the 8th or 9th to the 15th century, a period during which modern literature in Irish began to emerge. It stands as one of the oldest vernacular literature in Western Europe, with its roots extending back to late antiquity, as evident from inscriptions utilizing both Irish and Latin found on Ogham stones dating as early as the 4th century. The early Irish literary tradition flourished through the Medieval Irish period, and its literary output showcases a blend of indigenous storytelling, myth, and historical narratives. Notably, this period saw the development of a full-scale vernacular written literature expressed in a diverse range of literary genres.

According to Professor Elva Johnston, "the Irish were apparently the first western European people to develop a full-scale vernacular written literature expressed in a range of literary genres." A significant aspect of early Irish literature is the influence of loan words from other Indo-European languages, including but not limited to Latin and Greek. This linguistic exchange is evidenced in texts like Sanas Cormaic, a glossary dating from the 9th century that illustrates the assimilation of foreign words into the Irish language. Two of the earliest examples of literature from an Irish perspective are Saint Patrick's Confessio and Letter to Coroticus, written in Latin some time in the 5th century, and preserved in the Book of Armagh.

It is unclear when literacy first came to Ireland. The earliest Irish writings are inscriptions, mostly simple memorials, on stone in the Ogham alphabet, the earliest of which date to the 4th century. The Latin alphabet was in use by 431, when the fifth century Gaulish chronicler Prosper of Aquitaine records that Palladius was sent by Pope Celestine I as the first bishop to the Irish believers in Christ. Pelagius, an influential British heretic who taught in Rome in the early 5th century, fragments of whose writings survive, is said by Jerome to have been of Irish descent. Coelius Sedulius, the 5th century author of the Carmen Paschale, who has been called the "Virgil of theological poetry", was probably also Irish: the 9th-century Irish geographer Dicuil calls him noster Sedulius ("our Sedulius"), and the Latin name Sedulius usually translates to the Irish name Siadal.

Two works written by Saint Patrick, his Confessio ("Declaration", a brief autobiography intended to justify his activities to the church in Britain) and Epistola ("Letter", condemning the raiding and slaving activities in Ireland of a British king, Coroticus), survive. They were written in Latin some time in the 5th century, and preserved in the Book of Armagh, dating to around 812, and a number of later manuscripts. The 6th-century saint Colum Cille is known to have written, but only one work which may be his has survived: the psalter known as the Cathach or "Book of Battles", now in the Royal Irish Academy. Another important early writer in Latin is Columbanus (543-615), a missionary from Leinster who founded several monasteries in continental Europe, from whose hand survive sermons, letters and monastic rules, as well as poetry attributed to him whose authenticity is uncertain. The earliest identifiable writer in the Irish language is Dallán Forgaill, who wrote Amra Coluim Chille, a poetic elegy to Colum Cille, shortly after the subject's death in 597. The Amra is written in archaic Old Irish and is not perfectly understood. It is preserved in heavily annotated versions in manuscripts from the 12th century on. Only a little later, in the early 7th century, Luccreth moccu Chiara, a Kerryman, wrote poems recording the legendary origins of Munster dynasties, including Conailla Medb michuru ("Medb enjoined illegal contracts"), which contains the oldest surviving reference to characters and events from the Ulster Cycle.

The oldest surviving manuscripts containing examples of the written Irish language date to the 8th century. Their Irish contents consist of glosses written between the lines or on the margins of religious works in Latin, most of them preserved in monasteries in Switzerland, Germany, France, and Italy, having been taken there by early Irish missionaries, and where, not being understood, they were rarely consulted and did not wear out, unlike their counterparts in Ireland. They are thus quite different from manuscripts with significant Irish language content preserved in Ireland, the oldest of which is the Book of Armagh (c. 812). The early glosses, though of little interest outside of philology, show the wide learning of the commentators and the extraordinary development, even at that early period, of the language in which they wrote. Their language and style, says Kuno Meyer, stand on a high level in comparison with those of the Old High German glosses. "We find here", he writes, "a fully-formed learned prose style which allows even the finest shades of thought to be easily and perfectly expressed, from which we must conclude that there must have been a long previous culture [of the language] going back at the very least to the beginning of the sixth century". The glosses are to be found in manuscripts from Würzburg, St. Gallen, Karlsruhe, Milan, Turin, Sankt Paul im Lavanttal, and elsewhere. The Liber Hymnorum and the Stowe Missal are, after the glosses and the Book of Armagh, perhaps the most ancient manuscripts in which Irish is written. They date from about 900 to 1050.

The oldest books of miscellaneous literature are Lebor na hUidre, or "Book of the Dun Cow", transcribed about 1100, and the Book of Leinster, which dates from about fifty years later. These books are great miscellaneous literary collections. After them come many valuable vellums. The date at which these manuscripts were penned is no criterion of the date at which their contents were first written, for many of them contain literature which, from the ancient forms of words and other indications, must have been committed to writing at least as early as the 7th century. We cannot carry these pieces further back with firm certainty using linguistic methods, but it is evident from their contents that many of them must have been orally transmitted for centuries before they were committed to writing. A 17th-century manuscript may sometimes give a more correct version of a 7th-century piece than a vellum many centuries older.

The exact number of Irish manuscripts still existing has never been accurately determined. The number in the Royal Irish Academy, Dublin, alone is enormous, estimated to be about fifteen hundred. O'Curry, O'Longan, and O'Beirne catalogued a little more than half the manuscripts in the Academy, and the catalogue filled thirteen volumes containing 3448 pages; to these an alphabetic index of the pieces contained was made in three volumes, and an index of the principal names, in addition to some other material in thirteen volumes more. From an examination of these books one may roughly calculate that the pieces catalogued would number about eight or ten thousand, varying from long epic sagas to single quatrains or stanzas, and yet there remains a great deal more to be indexed, a work which after a delay of very many years is happily now at last in process of accomplishment. The Library of Trinity College, Dublin, also contains a great number of valuable manuscripts of all ages, many of them vellums. The British Museum, the Bodleian Library at Oxford University, the Advocates Library in Edinburgh, and the Bibliothèque Royale in Brussels are all repositories of a large number of valuable manuscripts.

From what we know of the contents of the existing manuscripts we may set down as follows a rough classification of the literature contained in them. We may well begin with the ancient epics dating substantially from pagan times, probably first written down in the seventh century or even earlier. These epics generally contain verses of poetry and often whole poems, just as in the case of the French chantefable, Aucassin et Nicollet. After the substantially pagan efforts may come the early Christian literature, especially the lives of the saints, which are both numerous and valuable, visions, homilies, commentaries on the Scriptures, monastic rules, prayers, hymns, and all possible kinds of religious and didactic poetry. After these we may place the many ancient annals, and there exists besides a great mass of genealogical books, tribal histories, and semi-historical romances. After this may come the bardic poetry of Ireland, the poetry of the hereditary poets attached to the great Gaelic families and the provincial kings, from the 9th century down to the 17th. Then follow the Brehon Laws and other legal treaties, and an enormous quantity of writings on Irish and Latin grammar, glossaries of words, metrical tracts, astronomical, geographical, and medical works. Nor is there any lack of free translations from classical and medieval literature, such as Lucan's Bellum Civile, Bede's Historica Ecclesiastica, Mandeville's Travels, Arthurian romances and the like. To this catalogue may perhaps be added the unwritten folk-lore of the island both in prose and verse which has only lately begun to be collected, but of which considerable collections have already been made. Such, then, is a brief and bald résumé of what the student will find before him in the Irish language.