The song that Desmond hears playing by the jukebox in the pub is "Make Your Own Kind Of Music" by 'Mama' Cass Elliot. It is the same song he played in the Swan station just minutes before Jack and Locke blew the hatch open.
The trivia item below may give away important plot points.
The conversation Donovan is having with his student before Desmond enters the rotunda echoes that of Charlie's eventual fate. (Desmond saves him many different times but the final outcome will always be the same)