Now known as, 'phtophthora infestans,' its spores would make their way to the Emerald Isle possibly by way of cereals imported from the U.S. or from bird droppings to be used as fertilizer from South America.
What made things particularly bad for Ireland was that in July the temperature suddenly fell. If it wasn't foggy then it was raining. A cold, wet damp set in. The conditions were ideal for the blight to spread like the virulent infection that it was and destroy the potato harvest. Soon the leaves would be black and the sustenance that people would survive on reduced to a chalky pulp.
The potato was introduced to Ireland from the Americas sometime around 1600. At first a garden vegetable, it grew in popularity to become a staple for the poorer Catholic lower class. It took little work and grew abundantly on small tracts of land. But having one major food to depend on would invariably lead to disaster.
There had been other famines before. However, most of these remained localized and didn't starve the entire nation. This was bad: In 1845 1/3 of the crops were lost and by 1846 it had grown to 3/4ths.
The British Prime Minister Sir Robert Peel was quick to act. He started public works projects to employ the destitute and brought maize in from the Americas. The political sentiment at the time, however, was laissez-faire and by June 1846 Peel had been ousted from power. Many of the British believed that things should be left to sort themselves out on the impoverished isle. Feeding the poor would only lead to more poor and then an even bigger strain on feeding all of them.
It is interesting to note that during the famine Ireland was still exporting about £100,000 of food to England every month.
Most of the Irish were little more than serfs who rented land from wealthy protestants who usually lived in England. They had lost their will because of generations of repression. If they had had a good harvest, the landlords would simply up their rent. Now, faced with starvation, many just gave up and died.
The population fell possibly by as much as 25% not just because of deaths from starvation and disease, but because of emmigration. Now it is safe to say that there are more Irish in North America than in all of Ireland.
And so was the worst time in all of Irish history.