I am very very tired. But here goes.
lenition across word boundaries: far from being evident in both older and modernised versions, except apparently anomalously in the phrase ‘a thige’ these occur only in the modernised version. does this date this 14th century text to pre-6th century or have i misunderstood?
This is a typical fudge on your part. 'a thige' isn't anomalous - it's exactly what you'd expect for Old Irish, where only lenition of c, p, and t was marked orthographically, just as we have here. (Though lenition of other consonants was pronounced too.) This is actually further confirmation - were it needed - that the text is Old Irish. You need to read the chapter on orthography in Paul Russell's book Vyv. This dates it to post 6th century (not pre- as you seemed to be arguing before.)
lenition intervocalically: i can’t find a single example of it in the older version.
the th in airthiur is an example (older *art -). Marking intervocalic lenition orthographically was a bit hit and miss in Old Irish: they could easily have written talchend; this is an example of scribal differences in Old Irish. If anything this helps us narrow the date to the earlier period, to around 750 (as I have argued all along.) Had you studied the matter, you would know this. that they chose not to suggests that the scribe was deliberately trying to give his newly composed verse the patina of age, the more to make it seem like a druidic prophecy. Old Irish scripts also had a lot of 'redundancy' provisions: i.e. when something was obvious to a speaker, they didn't always feel the need to mark it orthographically. A speaker would automatically have lenited the second element of a compound, so the scribe may have not felt the need to indicate this. This is quite standard.
the reduction of unstressed syllables: wouldn’t this have varied with locality as well as with time, some usages persisting in some dialects longer and others appearing earlier from place to place?
No. Old Irish has no dialects. It's one of the remarkable things about it - no trace of dialect at all. (From which we can draw certain conclusions about its sociolinguistics: about the kind of people who spoke it, and their intentions.) Had you studied the matter, you would know this. Besides, the loss of unstressed syllables (syncope) happened universally across all words as Primitive Irish turned into Old Irish. That, and apocope, are two of the major linguistic 'time' markers that help us differentiate different stages of the language. E.g Ogham Irish INIGENA 'daughter' turned into OIr 'ingen' with loss of the unstressed medial 'i' (syncope) and the final 'a' (apocope).
vowel affection to mark case endings: as you say this feature alone doesn’t date the text, but only places it later than 600, but since we already know that the copies we have of it are 14th century
Yes, but you wanted it to be earlier: specifically you noted that the time the prophecy is set would have been fifth century, and queried whether it could have dated from then. I was demonstrating that it can't be that early. I was showing a variety of pre- and post- cut off points as dating signifiers.
anyway, as i’ve already said we don’t know what linguistic features the original fáistine quoted in the copied text might have had – and you have not responded to that objection to your claim that it could be dated to ‘probably’ the 8th century.
The MS text IS IN OLD IRISH, the earliest stage of the language from which we have abundant texts. It is clearly a faithful copy of something written c. 800, because it shows linguistic features which indicate that date, and which cannot be much later, and equally cannot be much earlier.
in fact, megli, you have shown yourself so incompetent at identifying, let alone dating what you have shown me is a very significant passage from a very well-known text, that i can hardly expect you to proffer a scholarly opinion on the possible date of this modernisation.
Best ignored, I feel. [deeper sigh]
but can anyone tell me who gave it the ‘Fáistine Teachta Phádraig’ and when?
No. Why don't you - orginal idea here - do some research? Why not quote us your original source? why not post us the texts from Rawlinson B and the Egerton MS, and see if they supply that title? It would involve you doing some homework. If some modern editior has given it the title 'the prophecy of the coming of patrick' - so what?! I remind you of the MS, which says immediately before the poem that Loegaire's druids 'prophesied the coming of patrick, and this is what they said'. 'The Prophecy of Patrick's coming' is a perfectly reasonable title for an editor to give it this tiny lyric. But this has no bearing on the actual Old Irish text. Which I have quoted for you.
Vyv. May I remind you - and other readers of this thread - that you don't actually know Old Irish. You've never read a text in it. You've never studied the grammar. You have never taken a class in it. You are roughly in the position of someone with a working knowledge of holiday Italian (say) criticising the work of scholars working on the most difficult Latin texts, on the grounds that thei account of the grammar of the language is not identical with that of Modern Italian. As if that was some kind of surprise. Or more closely, in fact exactly, like a Spanish person who has learnt conversational Modern English, and then proceeds to tell scholars working on Beowulf or The Wanderer that they are profoundly mistaken, without bothering to learn any Old English, prostituting a wonderful text for a series of ahistorical and off-the-wall theories. It is a mind-boggling - and really rather sad - position to find yourself in.
Why don't you go away with Peter Schrijver's new book, or the Lehmann's Introduction to Old Irish, or Quin and Strachan's Workbook and Paradigms and Glosses, and learn the language you wish to discuss. Read fifteen to twenty lengthy texts in it. Beith and I will help you if you get stuck. That should take you several years, as it takes everyone: knowing some Modern Irish is an advantage. Then when you have a good mastery of the language, come back, and we will accept you as being compentent to discuss it.