I've heard this opinion expressed on a podcast without any information to back it up and it's pretty hard to look for.
Were there any great sources of calories that could have been extracted with the technology of the time and that could have sustained the entire population of Ireland?
EDIT: "Overlooked" was the wrong word. The idea is that there was some food that they didn't really know about.
I have never heard this before. There is a debate about whether the quantity of food exported could have replaced that lost from the potato. Ó Gráda argues that even if the government could have seized privately owned grain and provided it to the poor, it would not have been enough. He calculated that food production was reduced from 30,000 million kcal per day to 15,000 million kcal per day between 1840-45 and 1846-50. Given how desperate people became, it seems unlikely that they would have ignored any food source.
Sources
Cormac Ó Gráda, 'CENTRE FOR ECONOMIC RESEARCH WORKING PAPER SERIES, 2004, Ireland's Great Famine: An Overview'
You sure the guy wasn't talking about "A Modest Proposal"? :P http://art-bin.com/art/omodest.html
Cannibalism was an option that some people took as well...so I think it is fair to say that if something was even remotely edible, the people would have eaten it before cannibalism. I have heard about a type of seaside dune grass that has been proposed as a potential rice crop, but again, its likely that the Irish would have already been snapping it up along with anything else that looked like grain.