Iceland has always been big into fishing,it is the main source of "home grown" food. Why was this not the case with Ireland,especially since they were in a famine with a relatively steady supply of fish?
The Irish have a long-standing history of not liking to eat fish, despite living on an island surrounded by good fishing. Part of this is due to Catholicism regarding fish as fasting food, and part due to the impracticality of getting fish from coasts inland in good condition in the absence of good roads or other transport networks. However, that doesn't really explain the famine situation, which is a little complicated.
First, there definitely was fishing as a major source of food. In the period between the introduction of the potato as a staple food, and the advent of the first famines due to potato blight, a lot of the (huge) population of the country lived on potatoes, milk, and fish - mostly herring. The harbours were full of fishing boats, there were plenty of people going out fishing and so forth - but the issue is that that can't be ramped up at short notice to compensate for another shortage. The seas off the Irish coasts, much like Iceland, are rough, and need fairly solid boats before you can go out fishing in any safety. There's a strong, strong trend in Irish folklore that men die at sea, and you'll see that right up into early 20th century literature. In the meantime, the high population meant that a lot of areas of marginal land had been cleared of timber (for building and for firewood) in the previous century or so, and put under potatoes instead. So there wasn't even material to build more boats at short notice.
The other issue is that people - mostly - still had to work. The vast majority of people worked for English landlords, farming crops or livestock for export. Those crops and livestock were grown and exported through the famine period, and the fact the workers' own potato crops at home were failing didn't affect the fact that they still had work to do. So not only were there not materials for more fishing boats, there wasn't labour to build them, nor labour to crew them.
There's plenty of evidence that people engaged in on-shore fishing during the Famine, as well as gathering shellfish, seaweed, and so on. But there's a limit to how much can be got from the seashore, and it's very time-consuming work even it hasn't been picked over by the rest of the coastal population already. So that didn't bolster the food supply all that much either.
If you're interested in the Famine conditions, I recommend Cormac Ó Gráda's work, particularly The Great Irish Famine and Black '47 and Beyond: the Great Irish Famine in History, Economy and Memory