The IP trolls that were claiming copyright on it finally lost in court in 2014, so it’s clearly in the public domain in the U.S. now. It was not for a long time.

It’s kind of unfortunate, really, because it’s a terrible song, especially when sung off-key by all your friends and relations with your name distorted to fit the limping scansion. It was nice when no one sang it on TV, at least.