I still think it probable that M. Considine used a then known tune for his poem.
(1) How else could he likely transmit the tune for "The cross of Spacnelhill" in his letter except by telling which tune he meant?
(2) In "Ballads of Co. Clare" the tune "Spancelhill" is indicated for altogether nine local songs (if I have counted correctly) by different authors. The songs come from various times, all but one under the broad category "1850-1930". That means several authors have used this tune to carry their songs between 1850 and 1930. Some of the songs have names in them that could make a closer dating possible, but not for me.
If so many different authors have used this tune before 1930, I think it probable that a tune 'Spancelhill' was known in Clare in the 19th century and has been used by Considine to carry his poem.
For the fun of it, the first verse of the most recent example that uses the Spencelhill tune
The black Minorca cock
One night as I lay dreaming
of pleasant days gone by
my chickens and my rooster
were cosy, clean and dry.
Till Reynard came across the hill
my poultry to destroy
and to send me to thew storekeeper
my gluggers for to buy.
And an old example:
...I spied a fir clad hillside
on that wild and rocky breast
spread o'er by Cratloe's well kept woods
and it's blackthorn, oak and birch
and a pretty flock of colleens
came tripping in the rear
with their hurling team and their jackets green
the pride of Co. Clare.
Just to give you an idea how close and how different "The cross of Spancelhill" in "Ballads of Co. Clare" and Robbie McMahon's "Spancilhill" are: The number of verses is identical, the theme of each verse is identical (slighly different order of verses), but each line I have checked is different in detail. I post the last verse for comparison:
And when the vision faded the tears came to my eyes,
but I hope to see that little spot once more before I die
may the King of joyous angels his blessings ever spill,
on that masterpiece of nature the Cross of Spancelhill.
compare (copied from above):
But when my vision faded, the tears came in my eyes,
In hope to see that dear old spot, some day before I die.
May the Joyous King of Angels, His Choicest Blessings spill,
On that Glorious spot of Nature, the Cross of Spancilhill.
Even the long "original" version has already variants that must have been known in Clare long enough to allow changes.