While not the only cause, one of the largest contributing factors was The Great Famine, or The Irish Potatoe Famine that started in 1845. While the potatoe blight had happened all across europe, Ireland was affected much greater than any other European country, due to Corn Laws that the English passed, causing a great majority of the Irish peasents to be dependent on the Potatoe as their staple crop. It's estimated that Ireland lost upwards of 20-25% of it's population during the Famine to starvation and emigration, most of which were the working class, who usually keep a fuctioning Industrial economy going, and help it boom.

There's many many other factors, mostly cultural and the British's impact on the isles, but the Potatoe Famine is one of the easiest to see and talk about.

My source details the Irish economy extinsively iin the early and mid 19th Century and the impact the famine had on it. It's a bit dense, but a great read if you're REALLY interested in the time period for Ireland.

Source: Mokyr, Joel. 2013. Why Ireland Starved: A Quantitative and Analytical History of the Irish Economy, 1800–1850.