“The vocal section of the song is really short,  so that you can sing along, and the music section is really long so you can drink and dance in between.”

This folk belief is essentially based in the stereotype about Irish people: that alcohol, and drinking, are of the utmost importance.  This belief filters down to music, and to the belief that little eccentricities of Irish music, such as having short lyrical verses and long instrumental sections, are there for reasons of drinking, dancing, and other merriment.