I thought the following might be of interest to anyone patient enough to read my ramblings! I would welcome any comments, particularly from anyone more knowledegable than myself in these matters, as I'm a very amateur sort of scholar.

THE SCHOLARS' PRIMER
This work is important as it contains, so far as I know, the first historical reference to the association between the ogham and the set of trees -Silver Birch, Rowan, Alder etc - with which we are familiar.

The full title of the work, in the edition edited by George Calder and first published in Edinburgh in 1917 (as reprinted by Four Courts Press, Dublin, in 1995) is:

"Auraicept na n-Eces; The Scholars’ Primer: Being the texts of the Ogham tract from the Book of Ballymote and the Yellow Book of Lecan, and the text of the Trefhocul from the Book of Leinster."

The Book of Ballymote dates from 1391; The Yellow Book of Lecan was written between 1391 and 1401; and the Book of Leinster was compiled around 1160. In ‘The Scholars’ Primer’ George Calder collates the most important texts, which constitute an Irish grammatical and poetic treatise, the earliest parts of which are thought to date back to the 7th century CE. I assume that these texts were all copied manually, and on the whole the work is thought to date to the 12th century. According to Calder, the language is Middle Irish, but its basis belongs to the Old Irish period, and it owes much to the Latin grammarians. Ireland was apparently something of a haven for scholars fleeing various political and religious troubles on the Continent, which brought Irish scholarship to a very high point at an exceptionally early period.

THE LEGENDARY ORIGINS OF OGHAM: THE BIBLICAL ACCOUNT
There is, so far as I know, little or no trace of the Ogham prior to the 4th century CE. But ‘absence of evidence is not evidence of absence’, and the Primer states that the earliest inscriptions were on Birch wood. If their function at that stage was not commemorative or monumental, but served a more temporary purpose, they may not have survived.

The Primer itself contains two principal accounts of the origins of Ogham. The first relates the matter to Biblical history. After the fall of the Tower of Babel and the confusion of tongues, one Fenius Farsaidh, said to be of Scythian origin, sent 72 scholars to study the 72 languages which were then, according to this legendary history, in existence. From the 72 languages Fenius then selected ‘the Select Language‘, the language of the Irish, which was called Gaelic because the process was said to have been instigated by someone called Goedel, Gaoidel or Gaedel, after whom Gaelic and the Gael were named. “What was best accordingly of every language and what was widest and finest was selected for Gaelic…to wit, the one language that was more beautiful and excellent than any language...because it was selected from every language, and for every obscure sound of every language a place was found in Gaelic owing to its comprehensiveness beyond every speech.”

These claims are obviously exaggerated, and are evidently part of an effort to promote the Gaelic language as a worthy subject of study, rather than just a provincial curiosity.

The Ogham is also said to have been invented or 'discovered' at this time. Moreover, “for every sound for which no characters were found in all the other alphabets, characters were found for them in the Bethe Luis Nin of the Ogham.” Both the Irish language and the Ogham script are upheld in the Primer for their comprehensive modernity. “More definite than the Latin alphabet is this, to wit, the Beithe Luis Nin of the Ogham for it was invented last.” Its characters were named after the 25 noblest of the 72 scholars employed by Fenius. Alternatively, says the text, they were named after trees, “though some of these trees are not known today”.

THE TREES LISTED IN THE PRIMER
The text then goes on to list the trees, namely:

B: Beithe the Birch
L: Luis the Mountain Ash
F: Fern the Alder
S: Sail the Willow
N: Nin the Ash

H: Huath or Uath the Whitethorn
D: Duir the Oak
T: Tinne the Holly
C: Coll the Hazel
Q: Queirt the Apple

M: Muin the Vine
G: Gort the Ivy
nG: Ngetal the Broom, or Fern
STR: Straiph the Blackthorn
R: Ruis the Elder

A: Ailm, the Fir or Pine
O: Onn the Furze
U: Ur the Heath
E: Edhadh the Aspen
I: Ido the Yew

And the additional letters, sounds, or dipthongs:
EA: Ebhadh (also) the Aspen
OI: Oir the Spindle, or Ivy
UI or Y: Uilleand the Honeysuckle
IO or P: Iphin the Gooseberry or Thorn

WHAT IS 'EMANCOLL'?
Later, Emancoll is mentioned. This Calder translates as ‘Witch-hazel’. However, the Witch-hazel as we generally understand the term today is a tree native to North America and East Asia, and it seems unlikely that this is what was meant. Apparently ‘Wych-hazel’ is another name for ‘Wych-elm’, which is a species native to the British Isles, so this may be the correct tree. The letter stands for AE, or otherwise for double C, or X (which is seen as two Cs back to back, one reversed). ‘Coll’ is C, ‘Emancoll’ is double C, ‘eman’ evidently meaning ‘doubled’. As ‘Coll’ means Hazel, so it also seems possible that some variety of Hazel is the proper tree. (Wych/Witch in this context does not refer to witches the people, but to pliable branches such as coppiced Hazel trees produce).

NATURAL AND ARTIFICIAL 'WOODS'
“Now all these are wood names,” says the text, “such as are found in the Ogham Books of Woods, and are not derived from men“ [i.e. from the names of the scholars]…for it was “from the trees of the forest that names were given to the ogham letters metaphorically“. There are thus ‘artificial trees’, or ‘artificial wood’ on the one hand, and ‘natural trees’, or ’natural wood’ on the other, the former being the trees of the forest, and the latter the trees, or characters, of the Ogham. Moreover, the genus ‘wood/letter’ has three species, namely:
Principal wood: the vowels
Cross wood: the diphthongs
Side woods: consonants.

While there are a number of Oghams mentioned: birds, cattle, arms, rivers etc. it is evident from the text that trees have a priviledged position, due to the fact that the term 'wood' can apply both a tree and a character. This sets up a fascinating juxtaposition of natural and artificial, wilderness and civilisation. All the attributions would seem to have an alliterative basis.

THE LEGENDARY ORIGINS OF OGHAM: IRISH MYTHOLOGY
But back to the legendary origins of the Ogham. According to the first account, the Ogham was brought into Ireland from the Middle East, where, as I mentioned above, it was said to have been invented or discovered by Fenius Farsaidh after the fall of the Tower of Babel. According to the second account, however, it was invented in Ireland by Ogma, brother of Bres mac Elathan, the King. Bres was a King of the Tuatha De Danann, although his father was a Fomorian. In a rather obscure passage, the Primer states that ‘sound and matter were the father and mother of the Ogham’. Sound was its mother, because it was named after Ogma; matter was its mother, because the word was also derived from og-uaim, meaning “perfect alliteration…which the poets applied to poetry by means of it”, and also because “the hand or knife of Ogma” was the means of its inscription.

The Tuatha were of course a legendary divine people, but if an equally legendary date is to be put to the origins of the Ogham, this means that it was at some time prior to 900 BCE, when Ollamh Fodhla, of Milesian pedigree, is supposed to have become king, by which time the Milesians had already supplanted the Tuatha. If there is any sort of historical truth in the story of the Tower of Babel, then it was probably a Babylonian ziggurat, destroyed when Sennacherib sacked the city in 689 BCE.

A MORE DISTANT HISTORICAL ORIGIN?
Apparently, if we look for a place where the trees and plants mentioned in the Ogham tree count are truly indigenous, that place would be in central Europe, somewhere between the sources of the Rhine and the Danube. This is the place where Celtic culture is supposed to have had its origins. This suggests that possibly the Ogham may have had its distant origins in this area, or else the that bards and scholars working in Ireland were not taking a narrowly insular view when they compiled their list.

THE VARIANT TREE ATTRIBUTIONS WITHIN THE PRIMER
While the Primer is reasonably consistent in its attribution of the various trees to the characters, there are some variations as follows:
B: Invariably Birch
L: Rowan in the majority of instances; Elm also mentioned.
F: Invariably Alder
S: Invariably Willow
N: Ash in the majority of instances; Nettles also mentioned.

H: Hawthorn in the majority of instances; Test Tree also mentioned.
D: Invariably Oak
T: Holly in the majority of instances; Elderberry also mentioned.
C: Invariably Hazel
Q: Apple in the majority of instances; Rowan, Aspen and Holly also mentioned.

M: Invariably Vine
G: Ivy in the majority of instances; Fir and Corn(field) also mentioned.
nG: Broom in the majority of instances; Fern also mentioned.
STR: Blackthorn in the majority of instances; Willowbrake also mentioned.
R: Elder in the majority of instances; Rose (?) also mentioned.

A: Fir or Pine
O: Furze (Gorse) in the majority of instances; Ash also mentioned.
U: Heath(er) in the majority of instances; Thorn also mentioned.
E: Aspen in the majority of instances; Yew and Test Tree also mentioned.
I: Yew in the majority of instances; Service Tree also mentioned.

EA: Aspen in the majority of instances; Elecampane and Woodbine also mentioned.
OI: Spindle in the majority of instances; Ivy and Heath also mentioned.
UI: Honeysuckle in the majority of instances; Ivy also mentioned.
IO/P: Gooseberry in the majority of instances; Thorn and Pine (for P) also mentioned.
AE/CC/X: Witchhazel (An unlikely translation, as mentioned above; so Wych Elm; or perhaps some variety of common Hazel; ‘Doubled Hazel‘).