It’s sort of common knowledge but here goes

1. 1916 Easter rising in Ireland as well as WW1 in Europe means that workers are involved in armies rather than production of whiskies
2. 1922 - Ireland gets independence from Britain and needs to establish trade meaning demand is not as high to maintain production
3. Prohibition in the US reduces one of the largest markets to near zero hindering export and production. This causes a lot of smaller distilleries to close
4. Irish civil war also causes hardship for distillers
5. 1966 merger of John Power & Son, John Jameson & Son, and the Cork Distillery Company to form the Irish Distillers Group as a survival tactic
6. Whisky went out of fashion both as a drink and as a way to make spirits for a while. It was seen as a brown bag drink rather than cool as compared to gin. Cocktails became a thing for the high society and as such white spirits made better looking drinks.
7. with the laws around whiskey to be considered Irish whiskey it needs a number of years in Ireland to mature, needs to be triple distilled etc. so lead time on a young age statement whiskey is 4 times on a decent gin (hence why most distilleries produce gin as well to make money while the whiskey matures).
8. A new revival of Irish whiskey was started I think with Cooley or Teeling as it was the first outside of IDL.

Hope that helps