In General, the cost of fish.
The Irish Potato Famine had multiple causes:
The Cottier system was essentially serfdom by a more civilized name, plots of land deemed unprofitable for other use was used by farmers who rented out 1-1.5 acres yearly paid in labor. The crops of this land was largely in Potatoes, Oats, and flax. If the land was improved and the Farmer had an excess the landowner might pay an excess of cash for that year but it is likely that the next year rent would increase leaving no real incentive to improve the land itself. Thus you have a large population of Farmers living without any cash but instead entirely dependent upon the soil which they work. Immediately prior to the Famine 33% of the potato crop was used as fodder for livestock. (Just to say that the Irish were not literally eating ONLY potatoes.)
The potato monoculture refers to having no genetic diversity of crop leaving the substince crop of most farmers at risk to a diseased crop.
During the potato Famine Ireland was a net exporter of food to Britain. Irish labor was cheap and Irish merchants could sell their goods to the British for far more than they could on their own Island. A similar situation during the 1780s saw a ban on export of Irish goods, locally prices dropped and the local populace was able to afford food. No such ban existed in the 1840s.
So while the fisheries may have had the capacity to feed Ireland, no market forces were in place to allow them to afford to do so.