Where did the reliance on the single crop of potatoes come from?
In part, from the Irish tradition of inheriting land. Instead of keeping the tracts together, it was split between many children. They came to rely on a crop that could keep a family alive on a very small plot of land. The English were not blameless, there is very seldom a simplistic answer to a lot of problems of this magnitude, but the English did not carry all the blame.
Violent disruption can cause famine, but the quote was all, not some. And it clearly said nature did not cause any. I don't agree nature doesn't cause any.