"human population exceeds the sustainable carrying capacity of the planet"

The current population is close to 8 billion, but that is not relavant to things that happened around 1850 when the world population was around 1.2 billion.

Agreed... but only to the extent that humans on the whole had options - if they weren't squimish about displacing other people or exploiting new habitats. I threw that in as an aside because the issue of more mouths to feed than food to feed them is not confined to history... it is an on-going fact... not some abstract concept to be ignored. However, the Irish were living on an island, not wealthy and not in a position to pack up and leave at the drop of a hat. Plus they, or anyone else for that matter, had no way of knowing that this blight would far more devastating than any previously encountered. Experience isn't always the best teacher... because there is always that new experience to be had.