The trouble with calling it a famine is that it makes it sound like some kind of act of God, which it wasn't. There was plenty enough food in Ireland but it didn't find its way into the mouths of the peasantry. The trouble with calling it a tragedy, well, the same. "Tragedy" is one of those words we apply to air crashes or tsunamis. The implication, again, is that the thing was an act of God. Isn't God an easy bloke to blame?