"Agriculture" didn't fail.

The Irish were successfully farming a number of highly nutritious crops. It's just that all of those were taken by the English, under the colonial system of the time.

The potatoes were one of the very few things they were allowed to grow for themselves (and are a great staple crop! near-complete nutrition all in one handy package!). That's why they were left starving when the potato blight hit, not because they were pathetic unenlightened regressives.