I can add info about the history of Irish cuisine.

Prehistorically, excavations of sites dating as far back as 7000BC reveal trash heaps containing a lot of fish bones and shellfish casings. Waste from river animals such as salmon, trout, and eel are found dating to the later Mesolithic period, and a greater presence of sea fish like cod and ling find their way into the archeological record around the Bronze Age as the native inhabitants of the island mastered sailing.

The first big shift in aquatic eating habits came with the Christianization of the island, and the importation of meat-eating practices from regions with less ocean access. Specifically, the Catholic practice of eating only fish meat on Fridays and during Lent made fish become not just a source of protein, but a religious behavior associated with specific observances.

English conquest of the island again brought about significant changes to the Irish diet at the same time as a massive shift in societal structure. Previously, fishing practice was booming in the 15th and 16th century for Ireland - it was profitable to be a talented fisherman, as Europe was beginning to import fish from the Irish Sea. During this time Irish fishermen had state of the art fishing vessels and the ability to collect deep-sea fish varieties. In the second half of the 16th century, however, the Tudor conquest severely restricted ship ownership and commercial market access (along with other areas of life like religious observance and land ownership). At this point there grew a noticeable split in society between the landed Protestant elite, many of which were supplanted by the English crown, and the native Catholic Irish who were usually less well off. The upper and middle classes had a diversified diet that included fish bought at market or caught on their estate lands by hired fishermen. They also ate some shellfish such as lobster, crabs, and oysters as delicacies. The Irish poor, on the other hand, were forced to sell their most valuable foodstuffs for rent money and eat what was left over. Fish was still consumed, but only by those in the vicinity of water sources. The coastal poor also began to gather more shellfish, but not as a delicacy – they ate seaweed and mussels that could be easily gathered on the beach by women and children. This fare came to be known as bia bocht, or poor man’s food. Fish almost completely vanished from the inland diet, as most river space was now owned by landed gentry.

Around the time of the Famine, fish factories were set up that employed experienced fishermen, but these workers no longer had the ability to bring the fish they caught home, as it belonged to the factory’s company. River trout, salmon, and eel were also severely depleted by these unsustainable organizations, pushing the industry towards less profitable herring and deep-sea capture, the latter of which the Irish fishermen no longer owned the equipment to participate in, except in rare cases. Herring sales did make it possible for the inland poor to purchase some fish products, however, so a slight increase in consumption was seen in this population. Thus, island-wide, the Irish were certainly eating seafood, but only poorer selections of it, as a necessity, and in a way where they couldn't economically benefit from its consumption.

In the late 19th century, the quality of life improved for the poor that were left in Ireland, yet the damage to fishing habits was done. Canals, refrigeration, and trains now allowed the quick transportation of fish stock to the innermost parts of the country, but there was a deeply ingrained association of seafood with religious penance and subsistence living. Other foods on the other hand, like wheat bread, tea, and beef, were now available for purchase that had associations with security and wealth. Purchasing power was thus spent on these foodstuffs with fish relegated to mostly religious observance.

It is interesting to note that fishing as an industry took off at this point – there was great demand for fish from the Irish Sea across Europe now. State organizations were established to manage the collection and sale of seafood on the island such as the Irish Sea Fisheries Association, but nearly everything caught in Ireland was exported. On the eve of the Second World War, Irish fish consumption stood at less than 3kg per capita per year.

And now if you'll allow me one paragraph on modern fish consumption - ever since WWII, Irish domestic fish consumption has been steadily increasing. Immediately after the War, yearly per capital intake jumped to 5.1kg, and now stands around 16kg. Fish is eaten at least once a week by 44% of Irish people. While less than the European average, this is a substantial improvement in the rate of seafood consumption compared to past levels. The number continues to increase every year as well, as the country becomes more health conscious, less stringent about Catholic fasting tradition (although fish consumption still peaks markedly on Fridays), and less haunted by subsistence living. In addition, modern contact with foreign seafood cooking traditions is increasing demand. Salmon and cod, two local varieties, constitute 60% of the Irish fish market, but species like turbot, halibut, haddock, and tuna are making quick inroads. Many of these can be caught in waters surrounding Ireland, but have been popularized recently in French, Spanish, and Portuguese cooking styles. In fact, the increase in fish consumption is such that Ireland now has to import cod, because the demand exceeds Ireland’s allotted EU quotas in the Irish sea.

So in general, fish consumption in Ireland has mirrored broader societal changes. It followed fluctuations in religiosity due to its ceremonial use in Catholic observance; was in some ways an unobtainable luxury and in others a necessity during English occupation and famine; and is now diversifying along with the rest of Irish society and cuisine due to integration with broader European and world societies.

Sources:

• Clarkson, L.A. & Crawford, E.M. (2001). Feast and Famine: Food and Nutrition in Ireland 1500-1920. Oxford: Oxford University Publishing.

• Mac Con Iomaire, M. (2006). “The History of Seafood in Irish Cuisine and Culture.” Dublin Institute of Technology School of Culinary Arts and Food Technology Conference Papers.

• Sexton, R. (2005). “Simplicity and integration, continuity and change” in Culinary Cultures of Europe. Germany: Council of Europe Publishing.

Scottish medievalist here.

Scottish populations generally clustered around the burghs - large-ish settlements with trading privileges, usually on the coast or near a major waterway. Because they were near the sea, fish was an important food and trading commodity, but not the only one. Many people lived inland, and brought their pigs, sheep, and cattle to market in the burghs.

These were an absolutely crucial part of the trading economy, partly because of their versatility. Most animals had two key products - their meat and their hide. The meat was sometimes traded, but much was kept for local consumption. Hides, though, as well as the lucrative wool from sheep, were a major commodity underpinning much of the Scottish economy. Markets in Europe, particularly Flanders, would pay a lot for these materials, which underpinned their own industries, particularly in textiles. In addition, the Scottish climate was far too harsh to keep large herds of animals over the winter, so the cycle of slaughter and selling was built in to the economy. Work as a farmer or a keeper of animals would ensure you money from selling your wares, as well as a steady supply of food for yourself and your family to last you through the winter.

Getting back to fish, these were caught in large numbers. Herring, mackerel, and salmon were the main Scottish catches. River fishing for salmon was especially important. But fish was a far less versatile product than 'farm' animals, and so could not sustain nearly as large an industry. They were eaten, but there wasn't as much money in catching them. In addition, u/smbtuckma alluded to the religious importance of fish to the Catholic Church, and this is also relevant here. One of the more common grants Scottish kings and nobles made to abbeys and monasteries was fishing rights on nearby rivers. I mentioned the importance of salmon - in many places, the local religious house had a virtual monopoly on the salmon fisheries. Many of the fish they caught would be kept for their own consumption, others would be sold at market, but for the ordinary person they were very hard to come by, and impossible to catch for themselves without incurring the wrath of the monastery. Except for some coastal settlements where mackerel and herring were caught for subsistence, fish was not available in large enough numbers to be a staple food for the ordinary person.

As far as later centuries go, I can't speak with as much authority, but I do know that fishing as an industry was massive for many years in some parts of the North and East of Scotland, and from my own experience many traditional and popular Scottish meals do involve fish. Not to the extent of the Japanese, but fish is a popular feature on menus.

The answers seem to lean toward the Fish side of the equation.

Did Japan have a large herds of pigs or ruminants at the time? If not, was there some physical reason other than culture?