"I actually don't think that such internal evidence as there is supports your conclusion."
Why?
I've given you some examples - would you like some more?
"why do they only rarely contain information that would be limited to those communities, "
For the same reason they were missed in Ireland - collectors had a preconception of what a folk song was and went out to find songs that fitted what had gone before
THey sked for "old songs" yet the local songs didn't fit that description because many were recently made
Sharp actually wrote about not taking down local songs in some cases
Many of these songs, served as the folk voice for a short period and died when the memory of the events died
If you want some examples of Irish songs that were made locally try The Bobbed Hair or The Quilty Burning or The Leon or The Broadford Lads or Dudley Lee the Blackleg... or around a couple of dozen others HERE
This is one we missed putting up

That Cold Man by Night.   Martin Long, Tooreen, Inagh, Recorded July 1975 at Willie Clancy Summer School
The practice of young women being pressurised or even forced into arranged marriages of convenience to older men has inspired many songs throughout these islands; sometimes depicting the tragedy or resigned bitterness of the situation the woman finds herself in, but occasionally, as with this one, open defiance, with a touch of humour.
This appears to be a locally-made song; we have been unable to find another example of it outside Clare.
Particularly interesting is the description of the visit to the matchmaker (the “learned man”) and the celebratory ceremony to seal the ‘made match’.

I am a handsome comely maid; my age is scarce eighteen,
I am the only daughter of a farmer near Crusheen,
‘Tis married I intend to be before its winning daylight,
Oh, my father wants me to get wed to a cold man by night.

This man being old, as I am told, his years are sixty-four,
I really mean to slight him, for he being wed before,
His common shoes are always loose, and his clothes don’t fit him right,
Oh I don’t intend the wife to be of that cold man by night.

The very next day without delay they all rode into town,
To a learned man they quickly ran the contract to pin down;
Into an inn they did call in to whet their whistles nigh,
In hope that I would live and die with that cold man by night.

My father came, I did him blame and thus to him did say,
“Oh father dear, you acted queer in what you done today,
In the Shannon deep I’ll go and sleep, before the mornings light,
Before I’ll agree the wife to be of that cold man by night”.

“Oh daughter dear, don’t say no more, or be a foolish lass,
For he has a house and four good cows, and a sporting fine black ass,
He has a handsome feather bed where ye may rest by night,
So change your life and be the wife of that cold man by night”.

“Oh father dear, don’t say no more, for I’ll tell you the reason why,
Before I’ll agree the wife to be, I’d first lay down and die,
In the Shannon deep I’ll go and sleep before the mornings light,
Before I’ll consent to be content with that cold man by night.

My match is broke, without a joke, I’ll marry if I can,
Before (???) is over I’ll have a nice young man,
That will take me in his arms in a cold and frosty night,
And some other dame might do the same with that cold man by night.

It's a massive presumption to assume that they were 'desk-bound urban outsiders'"
It most certainly is not - it's a well documented fact, including in Hindleys Hindley in teh Catnach biography and Leslie Shepherd's books on the subject
Vic has described the pressure they worked under quite adequately
Beside the point anyway - they were hackneyed poets (Hacks) and their output is dry, brittle chalk to the rich-tasting cheese of folk poetry
If you want to spell out what differences have made Ireland capable of folk poetry and England incapable, please do
Alluding to them doesn't work in debates such as this
Jim Carroll