Áine's etymology sounds right to me. It'd explain why the word could mean a rope trick, and also a party - both involve confusion.

In the context of the song I'd have thought party/shindig makes more sense than rope-trick. "When this cattle trail is over, I'm going to have a high old time" seems more likely than "When I get where I'm going I'm going to practice this neat rope trick". Though I suppose they might be having somne kind of rodeo and he's planning to win the rope trick prize.

Incidentally - has anyone got any idea where that word I just used, "shindig", meaning party, comes from, and why?

(I think it was Carl Sandburg who first brought this song to popular attention, with the words given in Cowpie.)