It starts where it finishes, in a dead-end drone: a single accordion note that seems to refine itself, thin itself out, even as it goes nowhere and lasts forever. That the song was recorded in 1985 is a mere accident of history: It could have been written at any point in the past 200 years. It could have been written by nobody at all—by Anonymous or by some mystery of collective authorship. Acid like a ballad by Brecht and Weill, blunter than all but the most sawn-off punk rock, the late Shane MacGowan’s “The Old Main Drag” is as undeceived a statement of human despair as anything in the canon of folk music. MacGowan, who died last Thursday, recorded it with his band the Pogues, and if you ever saw the Pogues play live, you know that their fans were wild. They stomped and roared and fought and sang along and spilled their Guinness, turning every show, anywhere, into a punctually berserk rite of the Irish diaspora. And how they adored Shane, the out-of-his-head front …