I've heard it many, many times since I've moved here to Idaho. It's sung by Basque shepherds as their walk herd at night. I'm told that it keeps the sheep from stampeding, attempting to get back to Wales.

It may well be an old Basque-Welsh tune dating back to when they Celts emigrated from the Iberian Pennisula to Eire and from Eire to Wales (I can't spell Cymra reliably). I will, however, call your attention to the word "dum" being used repeatedly, and the city of Dumdum in India. I'm not sure that it's a coincidence!