The very first subject introduced to a Foclo, the first grade of trainee Fili poet in the old Gaelic literary order that turned out forty generations of poets from 5-17C; was the Ogham alphabet, and the very specific tho highly convoluted tale of how it came to be invented by its creator, Fénius Farsaid.
We learn the tale in the Medieval poet-training manual, Auraicept na n-Éces, Scholars Primer, a 12C compilation of four books:
... unique among medieval grammatical works in that it represents the earliest vernacular tradition in Europe. Its earliest ('canonical') parts date as far back as the 7th century. In its present form, it contains much ancient material relating to the Latin and Ogham alphabets, the nature of Old Irish and Latin gender, comparison, and declension.
The first of them, The Book of Fénius Farsaid, tells the foundation myth of the Irish language, and goes into great detail about Ogham.
His and the other Auraicept na n-Éces texts were decanted from the Book of Lecan, Book of Ballymote, and the text of the Trefhocul from the Book of Leinster, into English in 1917 by George Calder, under the title The Scholars Primer.'
Calder labelled it Handbook of the Learned, but a more literal translation of Auraicept na n-Éces - I was told by a senior Irish speaker at a Poetry Ireland / Éigse Éireann event in Dublin - would be something along the lines of 'the system/working methods of poetry/knowledge'.
The word Éces being one of the most ancient Gaelic words for Knowledge/Poetry. Root of the name of Finn Eces/Finnegas ('bright knowledge') , the druid who taught a young Fionn mac Cumhaill.
Forty generations of Gaelic poets began their seven-grade trek to becoming an Ollamh 'poetry professor' ---and contender for the Ollamh Érenn 'poetry professor of Ireland' top spot occupied by such learned (and forgotten) figures as Gofraidh Fionn Ó Dálaigh, poet to Maurice FitzGerald, 1st Earl of Desmond--- learning that Fenius created the Gaelic language on the plain of Shinar in Babylon (modern day Iraq), three decades after Babel's collapse, when the 72 dialects of humanity's shared languages were scattered, until being retrieved by seventy-two (named) scholars, under the co-ordination of Fenius, who spent a decade retrieving them.
From which he then created, experimentally deciphered or back-engineered Hebrew, Greek, Latin, and, finally, perfected all language into the Ogham form, and the earliest recorded alphabet that the Gaelic language and its subsequent 1200 year literate Fili tradition found itself on.
We are told by the 7C author of the Book of Fenius in the Auraicept na n-Éces, that the seventy-two named scholars who'd done the donkey work collecting the scattered languages from which he created Hebrew, Greek and Latin, asked that Fenius, 'select for them out of the many languages, a language that no-one else should have but which might belong to them alone. Wherefore on that account was invented for them the (Gaelic) Selected Language (bérla tóbaide) with its (five) superadditions.'
1 - Bearla na Feine, the language or dialect of fenechas law. A high level language of the educated that the system of entirely civil law was preserved in and used by Brehon lawyers and Filidh poets for official business like law, ritual and ceremony. Also the language in which Auraicept na n-Éces is written, as well as Táin Bó Cúailnge.
2 - Iarmberla - Glossed as 'the additional language' by Calder, but commonly called the Iron Language.
3 - Bearla an Eaderscartha, the separative language or dialect; The Language Parted among the trees. This is the famous Ogham, a language used for encryption and memory lists. There are numerous Ogham tables in the Book of Ballymote, all with different names and uses.
4 - Béarla Filidh - 'language of the poets'. The Secret Language of the Poets, the 7C text states 'sometimes known as the 'Dark Speech' because it obscures meaning through the use of kennings and metaphors. 'The Poets used this language to converse among themselves, in tests and initiations, in producing chants, invocations and satires, especially when they wanted to reserve their meanings to the learned only.'
5 - Gnaithbhearla, the customary colloquial language and dialect of the illiterate majority. The common language that serves everyone and what became Old and Middle Irish, and eventually Modern Irish.
Obviously of interest to the poets is Béarla Filidh, which a 13C Brehon lawyer explains to his pupils:
'The literary language whose thrust is not self-evident or superficial
and the noble reading aloud—for ardent judges and bards, they
are the keys which release locks.'
It was a cipher language in which a skilled poet could communicate with other poetry professors and poet-lawyers above the heads of everyone but themselves. Where every letter was measured and elegant as sun-polished blackthorn blossom, their text communicating a multiplicity of meanings, the truest of which could be hidden in plain sight in words carefully selected and wrought to form the abstruse stream of Béarla Filidh, where every connection ---as John Minahane points out, quoting from a Latin Grammarian, in his groundbreaking work of scholarship and innovation: The Christian Druids: On the Filid Or Philosopher-poets of Ireland--- reveals 'knowledge of a thing (that) will die unless you know its name.'