Wikipedia reminds us that the Great Famine occurred between 1845 and 1852. Ireland's population decreased by 20 to 25 percent, half because of starvation, and half due to emigration.
It was similar to one of the Jewish diasporas - where the population is dispersed to the rest of the world, and effectively spreading the culture. Although the trigger was a bacterium, the fundamental problem was that over half of the Irish farms were not large enough to grow any crops but potatoes.
As explained in, Scientists Find Weakness of Infamous Blight-Causing Microbe, explains the Phytophthora infestans actually crossed the Atlantic, originating in Mexico. For the two years earlier, potato crops in the U.S. were devastated by the bacterium, but because of the Irish socio-economics, it was much more serious in Ireland.
The Popular Mechanics article tells scientists have identified the bacterium has two separate regions in its genome that permit it to adapt to new environments, permitting it to target a completely different family of plants. One region changes very slowly - giving the microbe hardiness, but a second region of the genome changes very quickly, increasing its surivability. Variants of the bacterium are known to attack lima beans, tomatoes, and even flowers such as morning glories. But now that researchers know they must attack both regions of the genome, they should be able to prevent future famines.