Will Fly,
First my apologies for not responding to your question earlier - a rather pleasant family visit prevented me from becoming more than superficially involved over the last few days.
Your original question:
Of course the traditions have taken up material of known authorship; but how much of this has happened recently has depended entirely on the health of the particular tradition.
MacColl's 'Freeborn Man' was, in our experience, extremely well received by the Travelling community, but whether it went into their song tradition is another question entirely. The 'versions' we have recorded from Travellers have, without exception, been either straight, unaltered repetitions or garbled fragments of MacColl's original. The song was written in the early 60s when the singing traditions in these islands (including among Travellers) was very much in decline and being remembered rather than still active and creative, therefore it was unlikely that new material was being absorbed and adapted.
There certainly have been cases in the past of songs of known authorship being filtered through the 'folk process' - Brian 'na Banban's' (Brian O'Higgins' 1882-1949) 'A Stór Mo Chroi' probably being one of the best Irish examples. The songwriting tradition in this area of West Clare was a particularly rich one and we have recorded dozens of locally made songs. One of the peculiarities we have noticed about nearly all these has been that, despite the fact that many of them must have been composed during the lifetimes of the singers we got them from, it has been virtually impossible to find the names of the composers - it appeared to have been a totally unimportant piece of information (this includes two songs where our singers were present at their making). Maybe this is due to the fact that there was no money involved in their composition - money certainly seems to play a part in the composition and ownership of songs nowadays - unfortunately. In my opinion attaching a name (and a price tag) runs contrary to the spirit of 'folk', 'tradition', however you care to identify it and guarantees that it will always be the property of the composer rather than the folk (in the proper sense of the word) and will remain unabsorbed and unadapted.   
So a shorter response to your question - if the machinery was in place and in good condition, it doesn't matter where the songs came from for their acceptance into the tradition.   
Regarding your point about tunes (sorry to go on at such length), I think the same applies.
Carolan is a difficult one; there is certainly no doubt that some of his compositions were taken up by traditional musicians, but I am not sure they have been adapted - they have always sounded (to my uneducated ear) to be composed pieces, somewhat grand and stately, and have stuck out as such among the rest of the repertoire.
I may be totally wrong, but I have always thought that the introduction of Carolan's music into the Irish scene came from Donal O'Sullivan's book 'Carolan; the Life, Times and Music of an Irish Harper' which was published in 1954.
On the other hand, you don't have to go back centuries to find known composers material going into the Irish traditional repertoire. It is full of such pieces; Reavey's, O'Dwyers, Cooley's, Morrison's... all of which have been absorbed and have taken on regional adaptations.
Again locally to West Clare, the best example of this has been the compositions of the blind itinerant piper Garrett Barry who was playing and composing around the beginning of the 20th century and whose music is very much a part of the local tradition.
I think that there is much more to be said on this fascinting subject (thanks for that Will), which involves (again) the definition and (rarely discussed) function of folk song (proper) and music in our culture and social history - but maybe I'll go back to bed.
Jim Carroll