Can't add much to the above, except that the set of words beginning

"One morning early, as I roved out
By the margin of Lough Leane..."

is a translation/version of the Irish "Fainne Gael an La

"Ar maidin moch do goes a mhath
Ar bruach Lough a Leine

(Sorry, memory fails here)

I don't think this was the set recorded by McCormack, but rather:

"One morning fair, as I walked forth
Where Leane's dark waters flow,
When summer bid the groves be green,
The lamp of light to glow;
As on by bower and town and tower
And wide-spread filds I stray,
I met a maid in the greenwood shade,
At the Dawning of the Day"

In the 1930s and 1940s every schoolchild in the Republic knew the air, at least, and maybe the Irish words. That P.K. intended his words to be sung to this air seems incontestable.