I am surprised that there is no mention of the Bay side hum in San Francisco. I am of the opinion that this is under ground tunneling that is happening or mining. That could be either the government or a private company doing this. Obviously it is of a clandestine nature and no doubt it’s under tight security. Which makes me think of a government program like military installations under ground. In WW2 the Royal Air Force had small underground facilities in Scotland that are still there. Usually in the countryside out of view. The only tell tale sign was a little metal fence and a manhole. I was on a school excursion once as a teenager and we were looking at historical sites where one of these Air Force bunkers was a right next to it. Our teacher pointed it out to us and I recall seeing a documentary on these Air Force under ground installations around that time in the late seventies.
So it makes sense that this could be what is going on with these strange noises not just in Ireland but also around the world.