Mentioned in passing in the 9th century Sanas Cormaic (good forgery if you can make it).
Let's see how difficult. Everything from Wiki
Sanas Cormaic (or Sanas Chormaic, Irish for "Cormac's narrative"),[1] also known as Cormac's Glossary, is an early Irish glossary containing etymologies and explanations of over 1,400 Irish words, many of which are difficult or outdated. The shortest and earliest version of the work is ascribed to Cormac mac Cuilennáin (d. 908), king-bishop of Munster.
OK, so it's supposed to be late ninth century. But see later.
It is an encyclopedic dictionary containing simple synonymous explanations in Irish or Latin of Irish words. In some cases he attempts to give the etymology of the words and in others he concentrates on an encyclopedic entry.
Pretty impressive for the ninth century. Our own first efforts were Doctor Johnson and others in the eighteenth century.
It is held to be the first linguistic dictionary in any of the non-classical languages of Europe.
Even more impressive!. Actually I'm not entirely sure that there is anything like this in the Classical languages (someone please advise).
Numerous of its entries are still frequently cited in Irish and Celtic scholarship.
You betcha. So when was it produced? The authorative source I take to be Manuscript A which seems to be from 1230. (Still damn impressive)
Leabhar Breac (Stokes’s version A) = Dublin, RIA, MS 1230 (23 P 16), pp. 263–72.
but it's based apparently also on this, for which we have a slightly later 'copy'
Yellow Book of Lecan (YBL) (Stokes’s version B) = Dublin, TCD MS 1318 (H.2.16), cols. 3-87 (= pp. 255a-283a in facsimile edition).
There seems to be seven early versions knocking around from the same sort of period. The only actual ninth century 'thing' we have is
MS Laud 610 (Stokes’s version G) = Oxford, Bodleian Library MS Laud 610, f. 79r-84r. Fragment corresponding to YBL, 756-1224.
This would indicate eighth century but on what basis I don't know. Shame it's only a fragment. Although it's also slightly strange that it's only a fragment since it must have been complete in the thirteenth century for it to have been copied. The copies seemed to have survived with no trouble but ... oh, no ... the original didn't. Normally I would assume all this is a product of 18th/19th century Irish nationalism but there being so many copies militates against this somewhat.