First, it should be noted that the potato is not a plant indigenous to the island of Ireland. It was introduced there and elsewhere during the reign of Elizabeth I, supposedly by Sir Walter Raleigh.
Before the potato came, other crops were grown, obviously. Grains were grown after the arrival of the potato, too. Remember, we talked about the difference between the larger farms, run as joint enterprises by the mainstream tenet farmers and the landowners. Remember also that the landlords were often nobility who had perhaps come into the land by way of grants from the crown, or bought as investments by their wealthy ancestors, and sometimes these landlord-owners were absentee. Remember also that the potatoes were grown as the main or sometimes only crop by the lowly farm laborers. Not everyone ate only potatoes by any means.
The potato caught on because it grew well in Ireland's mild and damp climate, grew well on marginal land (which Ireland had an abundance of), were tasty, nourishing, pigs liked potato peelings (still do) and, perhaps most importantly, one didn't need to be a rocket scientist to learn how to grow them successfully.
Of course, the very poor were not the only group of people who liked to eat potatoes, so they were also grown on the main farms as cash crops and for local food. However, potatoes weren't the only crop grown on the main farms by any means.
The farmers grew enough potato acreage to use horse-drawn farm implements for the planting and harrowing, and the farm laborers were there to weed and dig them in the fall. The laborers on their small plots of land, however, grew only enough to feed their families and perhaps enough to sell to buy a pig now and then or to purchase supplemental food for the hungry summer between potato crops. Because of the much smaller plots, the potatoes were planted by spade. That is to say, they pushed a spade a few inches into the ground, pushed it forward and dropped a seed potato piece behind the spade, then simply pulled the spade out of the ground leaving the seed buried. Fertilizer was simply farm manure and/or perhaps seaweed, depending on where they lived. Life was not exactly good, but there was survival and Ireland's population managed to double from 4 million to 8 million, largely because of the potato. Well, not ONLY because of the potato, but you get my drift.
Obviously they had experienced blights to their crops before over the years, but these had been localized and didn't happen every year, and didn't affect entire crops. These blights happened frequently enough so that none of the peasants ever got rich or overweight. In retrospect, one would have thought that the landowners would have done a bit more research into investigating these warning blights, but since the crops were mostly always better the next year, apparently they didn't bother thinking about it too much. Some men did, but were not paid much attention. If it's not raining, you don't worry about fixing your roof.
Potato blight is caused by a fungus. Fungi are propagated, often, by wind-borne spores. These spores attach to and begin "infecting" the potato plant's stalk and the tubers themselves. Since these parts are underground, the farmers didn't know (at least at first) until they began the harvest and saw the blighted tubers when they dug them. It didn't take the farmers long to identify the blight by the leaves discoloration and the mold on the underside, but since this is a symptom of the disease and not the cause, that was too late.
Here we should point out that potato blight is common worldwide, right up to the present day, a drawback of a moist climate, mostly. It was sad, though, that it wasn't until 1880 that agricultural scientists discovered that a simple solution containing copper sulfate sprayed on the stalks effectively repels the spores.
Another reason for the large failure was the continual growing of the same variety of potato over and over each year. In the plant kingdom, as in the animal kingdom. interbreeding is never conducive to good genes, and the plants' natural resistance to disease was lowered. They DID know about other varieties of potato, but grew the one which produced the largest yield.
Most historians believe that the blight came from the American continent, probably from diseased potatoes in ships from South America also carrying cargoes of guano. Guano (bird manure) had recently been discovered to be, and was prized as, a superlative fertilizer. I don't know where they got the guano. The only guano in any quantity I know of is "mined" from caves where bats live. Another fine job, I'm thinking.
The blight showed up first in Belgium, they say, then to the Isle of Wight (air-borne spores) and very soon thereafter to southern England proper. This happened in only a matter of months, of course, and it was in September of 1845 the the disease was verified in Ireland.
The first year of blight, 1845, saw a third of the crop unfit; later years were much worse. In 1846 only a quarter of the normal crop size came in edible, and the next 2 years saw only about a third of the normal crop. People on the edge quickly began to starve. The relief response by Britain and the rest of the world was eventually pretty massive, but not immediate.