Dan's praise is hardly justified; there are many of us who bring special knowledge and interest to bear on these problems. I especially appreciate Bruce Olsen, Sandy Paton and Abby Sale - as well as Dan himself - and others. My advantage is that I can draw on Steve Roud's indices, a large personal library and the resources of the Irish Traditional Music Archive; but thanks, Dan!
Abby's suggestion opens another can of worms. The song, "Johnny Johnny" he mentions is closely related to all the "O Waly, waly" but also connects with all those songs which begin "Love is pleasing ..."
Also "Caledonia" does seem connected. It seems to me that we would be well served if someone would make a compendium of all the songs which share "floating" verses and begin to identify patterns. The basic unit of learning and the basic identifying term in what we sing and what some of us study, is the "song." It seems to me that in lyrical songs where verses interchange, land and borrow, the unit of learning may be the verse, and the song is only identifiable from the particular verses of which it comprises.
Typical of me to raise a question and then run.