Having originally heard the song "The Lish Young Buy a Broom" from the same source as MtheGM I had no qualms about the word in question since being more familiar at that time with "The Keel Row" which too is from the North although the opposite coast to LYBAB the second verse goes:-
'O wha's like my Johnnie,
Sae leish, sae blithe, sae bonnie?
He's foremost 'mang the mony
Keel lads o' coaly Tyne;
Which I still maintain is a similar description (lithe, able bodied etc).
In the early seventies I once heard "Martin Said To His Man" introduced as an Irish song and more recently saw a CD "The Songs of Ireland" or similar which contained "The Fields of Vietnam"