The famine started 180 years ago, and the population fell by as much in the 80 years after the famine than it did in the years of and shortly after the famine. Going from over 8m before the famine to around 6.5m following it. But up to the mid-1920s the population continued to drop, right down to less than 4.3m. The famine killed roughly a million, the main population drop came from emigration that continued for almost a century (from the whole island, it wasn't until after WWII that population numbers began to rise in the republic).
The famine itself wasn't the reason the population numbers fell so low. It's because the country was completely fucked after it, and created a vicious circle, where things were so bad, people kept leaving, which made things worse, so more people left. That's not comparable to the current situation in Western countries. Our populations are dropping because of problems that could be fixed within a decade or two if the will existed.