I have gathered that the relaxation of the 'Corn Laws' was instrumental in providing alternative food sources to the Irish population.

Possibly a year off from growing potatoes, and then not growing them as intensively as previously (due to reduced population) will have reduced the hold of the blight.

Given that it wasn't the entire population of Ireland that depended on potatoes (those on the coast ate fish, and the richer farmers in the east grew other crops), and the very large population decrease, I expect that the number left of the potato dependent population segment was actually very small.