There were multiple reasons for both the Irish dependence on potatoes as a staple and the horrors of the Great Famine of the 1840s. One of the keys to understanding the issue is that, while potato crops were severely impacted by potato blight, there was still a lot of food being produced in Ireland throughout the Famine. This has led to accusations of mismanagement and even genocide perpetrated by the British government. u/locksymania wrote about these issues a couple years ago, and both u/InstantCoffee and u/Balnibarbian discussed this in another thread.

u/GothWalk and u/Foldweg also shed light on foodways in Ireland prior to potatoes being introduced, which might be of some interest.