I found this about the writings and why Celts passed things on by using oral history at first.
The earliest Irish authors
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 fourth 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.
[1] 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.
[2] 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 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.
[3] 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 the
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.
[4] 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.
[5]