Well, that is what they did just about

Except they didn't.

Not only are people not heavy (technically "dense", actually) enough, but they'd need to be right down in the bilges, which is where ballast goes. You couldn't put people in there for the obvious reason that they'd drown. Also, even assuming that this was possible, people can walk about, which makes them unsuitable for ballast which has to stay where it is to balance the boat.

This is clearly fakelore of the kind "we wuz so poor when we wuz kids, we had to share one shirt between the ten of us". Except that sadly some people have started believing it. I think the Irish had enough crap happen to them without inventing more.

Graham.