This episode marked David Hartman's first appearance as a series regular.
According to Wikipedia, The Irish Rovers is a group of Irish musicians, half of whom now live in Canada. Formed in 1963 and named after the traditional song "The Irish Rover" they are best known for their international television series, contributing to the popularisation of Irish Music in North America.
During the dance scene, the Virginian is not present. But as everyone goes out the door, the Virginian appears in the next scene, as if coming from the dance scene.