Karl Marx uses the Irish famine to show that capitalists are not interested in increasing output per se, only in increasing that output over and above the labourers consumption fund that falls to them (in this case as rents). He shows that in Ireland the landlords forced peasants to give way to pasture farming because even though the total output (peasants own consumption + rent) was less, the profits were greater than the rents. Hence their continuous pressure on the peasantry, reducing them to potato dependency. But again, this is not so much free market as in the text books, more like capitalism as it really is, wholly dependent on colonialist domination.