old irish was one of the first and oldest written languages in europe. most of what people think of as old irish is actually the literature of late medieval irish and not the old irish. all the gaelics are dialects of old irish, not separatr languages as some say. my gaelic isn't so good but when i was out on the west coast of sithern irish there weretimes i understood basic things better tan the local english. like doractions on the roads and how ro find food drink and a "rest station. which musch surprised me.
there are existing manuscripts from the 9th century, i believe but i would have to look that up. ducuils may be the oldest, again i'd need to research ot. however it's known that the copyists were already recopying from the time of christ for some works, and probably earlier for a few.
after the celtic church was established, they were the educated and literate people in europe out side byzintine territory. they were the denizens of the scriptoriums , so familiar in myth and fact. the irish monks(or those trained in the irish tradition) communicated with each other over thousands of miles and decades byt writting "glosses in the margins of the manuscripts. this is how the oldest irish was rediscovered. theirneissen, the great german philogist, collected all the glosses, and reconstructed the language. with many students they were able to get a complete picture of the language. these copyists wrote poems, a few phrases of music, gossip and theological discussions. most of these monks were dstill pelagians, a catholc heresy, so they hid it in the gaelic glosses.
these copyists were well established in ireland in ad 200, in iona by 450, in lindisfarne(holy island and its branch harrow) by ad 600, across central germany italy by 900 to 1100, all the way to slovenia,where the poem panguar ban was written.
one of the oldest tales gives exact and detailed discriptions of a chariot used circa 1800 bce by central suropean gauls. a chariot style never seen in ireland . showing the teaxts preserved details millenoium old.
the earliest gaelic is found on ogham on stones. hard to date bu there are attemps/
i just had a operation, but i'll try to look up the oldest known irish writing.
the older the writing the more complicted the grammar. the oldest versions have a case and mode system much more complicated than latin,if any of you remember latin class. this allowed the same meaning to be written in about a third the space--very good when you are writting on stones, and allowing poetic complications not easily understandable to anyone who is not familiar with cases and modes. scotts gaelic preserves some of this but not much. an example is you don't have to say "he said " or she said" in scotts, the case change tells you when a woman or man is speaking. you don't have to explain deferential and bowing down talk, the case change tells you you are defering and speaches by the powerfull tell you by case not extra words the speaker is powerfull.
this gets very very complicated in the oldest irish.
so in a general answer, theoldest irish was mostly written down by irish chuch monks from the second century. possibly by druids of the same era. many manuscripts were altered to conform to christianity but a lot is preserved. i would guess it was written quite a bit from the trun of the centuries from bce to ad