The irish obviously got their food from somewhere before, why didn't they keep some or all of these sources if they worked before?
There were major famines before the introduction of the potato caused by factors such a failure of grain harvests or deliberate destruction of crops during war. In part the potato was introduced in Ireland, as in other parts of Europe, as being a more reliable and nutritionally dense crop which would protect against the threat of famine. Unfortunately this strategy did not work. Population growth and over reliance on mono-culture meant that food security was perilous and once the potato blight set in the effects were compounded by poor, sometimes outright inhumane, governance.
The potato famines get much more attention than earlier famines because of the greater number of people effected and because of their historical contexts.
http://www.history-magazine.com/potato.html
http://www.theirishstory.com/2012/01/03/war-and-famine-in-ireland-1580-1700/#.Vk-bNNLhDIU
The introduction of the potato contributed to a huge increase in the Irish population. That increased population could not be fed by the crops Ireland used to produce before the potato.
The population growth in Ireland was not as great as that in England and Wales over the period, but England and Wales were in the midst of the industrial revolution, and urbanizing more rapidly than Ireland. The export of industrial products meant that England could afford to import food to feed the population. Ireland could not. The population growth in Ireland was about 50% more than that in Scotland over the period.
The population increase in Ireland was presumably enabled by the introduction of the potato as a crop and food source.
In 1740, the population of Ireland was 3.25m.
In 1841, the population of Ireland was 8.2m. The population of Ireland had grown by almost 5m people, 152%, in 100 years.
http://www.grantonline.com/grant-family-genealogy/Records/population/population-ireland.htm
The population of England was 5.8m in 1750.
The population of England was 15.9m in 1841.
The population of England grew by 174% in 90 years.
http://www.visionofbritain.org.uk/census/SRC_P/6/GB1841ABS_1
The population of Scotland was 1.3m in 1755.
The population of Scotland was 2.6m in 1841.
The population of Scotland grew by 100% in 85 years.
http://en.wikipedia.org/wiki/Demographic_history_of_Scotland#/media/File:ScottishPop1801-2011.jpg
The high population of Ireland in the early 19th century may have been an anomaly. The introduction of the potato as a staple crop had enabled an increase of the Irish population (though most were very poor subsistence potato croppers) which only lasted 100 years before a fungal disease attacked the potatoes, causing the potato famine, and an Irish population decline from which Ireland has still not recovered.
The combined population of the Republic of Ireland and Northern Ireland today is 6.4m.
By 1931, the population of England and Wales had grown to 40m, (400%), the population of Scotland had grown to 4.8m (270%), and the population of Ireland had shrunk to 4.2m (-46%).
The causes were:
The potato famine (which resulted in 1m deaths) in Ireland, and then decades of emigration from Ireland (much of which to England).
Certainly the Irish population has never recovered to what it was before the potato famine. The potato allowed Ireland to become very densely populated for a largely rural economy, and vulnerable to high dependence on a single crop.
To give a context to past Famines, the 1740 famine which was part of a Europe wide cold snap killed a higher proportion of the population of the Irish Population than the Great Famine a hundred years later. Frost killed the wheat, oats and barley and disease and starvation resulted.
Basically the potatoes increased the amount of calories that could be produced per acre of farmland. The increased yield lead to a increase in the population. The potato famine killed a large portion of the food supply. I'm not sure if people kept trying to plant potatoes and were unsuccessful or if when they switched back to growing grains there still wasn't enough of a yield to support the increased population.
