I tend to follow the reasoning of those with a working knowledge of the Irish. But I think the explanations here, taken within the context of the song, only allows two explanations. First, the song Little Beggarman, or any of the other names it goes by (Red-Haired Boy) mentions "with me pack on me back." so out goes the explanation that the rigadoo is a stick used as a carrying device. And to help out those that want to know how do you keep it on your back, watch the 1922 production of Nosferatu, and you'll see that a pack was actually two packs, or sacks, tied together and slung over the shoulder. This brings us to three explanations: Walking stick, dance, or drum. Where he "slips around the corner with his auld rigadoo" could easily be a dance step, but most versions I've seen say "Singing skin a ma rink (or link) a doodle on me auld rigadoo." To sing on something would likely indicate an instrument of some sort, and the drum fits here. So until we find this guy, we won't really know. Got a time machine? Maybe an Irish nursing home.