Why did Ireland's population drop after 1840?

Answer 1

A million died in the famine which followed the potato blight of 1845, and millions emigrated in the following century as landowners switched from arable to livestock farming, largely for the British market.

Answer 2

Before the famine in 1845, the population of Ireland was estimated at about 8 million. Many died and many emigrated, reducing the population significantly. It is estimated that about 1 million died and 1 million emigrated. The population has increased since then, but even now, it has still not reached 8 million again.