Hamish - there's a free MP3 of the entire song (~3.25Mb) at Tara's Thistle - free downloads from their albums (it's the third item, but I've given the page address in case there's anything else that takes your fancy). If I get time I'll put up an abc of the tune later.

On the other use of bummer shore cited above, I think there have been many discussions of it in the various Days of Forty-Nine threads and you can probably find all you want to know about it there (I haven't rechecked that so don't blame me if my failing memory is wrong about that).

My historical slang dictionary has a few entries for bummer (which has been around a bit longer than you think Hamish!), but none relevant here. I'm inclined to go with Inukshuk above, that it's related to boomer.

Mick