From a naval I point of view I find this idea rediculous. There are two uses for ballast.

The fist is to balance a new ship so that it is upright and stable. The second use of ballast is to keep the ship in correct trim once the cargo is loaded. The first sort of ballast is generally permenant and physically fixed to the ship - the second was loaded and moved around the ship as required.

Roughly speaking for a ship to be stable the centre of gravity of the ship needs to be below the centre of gravity of the water the ship displaces - the greater the hight difference the greater the stability. As has been said above this necessitates the ballast being as dense and a low as possible in the ship - you just couldn't get the required mass of Irish immigrants low enough in the ship.   But above all the one thing you don't want at sea is for the ballast (and cargo) to move about as this can catastrophically affect the stability of the ship. So unless you packed them in like sardines in a tin and prevented them moving for however long it took to make the trip then they would be useless as ballast.

Personally I think that this myth has arisen from misunderstood naval terminology.