The song makes perfect sense to me (even if some of the slang is vague). Boy courts girl, 3rd party joins the triangle, seduces her in Sandymount, First party gets upset and kills the third.

Still we'll always have "parrots" as Kryten would have said.

And I think that "midge" is more than likely original and "image" is the mondegreen, but each to their own.